Q: How do I clean my iMac OS

Don't install any of the so called "cleanup" apps. They are all worthless. They cause more problems then they are worth.

Also whatever you do done install any AV software either they are worse.

Use the security tools that included with OS X instead.

Allan is absolutely right.   You should not need any kind of 'Cleaner'   Just browse with common sense.   Even if you are careless, viruses as such are unlikely to take hold of your system, but as you will be aware, better safe than sorry.   Your Mac has more than adequate built in protection.

Do not download or pay for so-called "cleaning" utilities. They are not needed and can cause more harm than good. Macs are not the same as PCs.

Kappy's Personal Suggestions About Mac Maintenance

For disk repairs use Disk Utility.  For situations DU cannot handle the best third-party utility is: Disk Warrior;  DW only fixes problems with the disk directory, but most disk problems are caused by directory corruption. Drive Genius provides additional tools not found in Disk Warrior for defragmentation of older drives, disk repair, disk scans, formatting, partitioning, disk copy, and benchmarking. 

Periodic Maintenance

OS X performs certain maintenance functions that are scheduled to occur on a daily, weekly, or monthly period. The maintenance scripts run in the early AM only if the computer is turned on 24/7 (no sleep.) See Mac OS X- About background maintenance tasks. If you are running Leopard or later these tasks are run automatically, so there is no need to use any third-party software to force running these tasks.

If you are using a pre-Leopard version of OS X, then an excellent solution is to download and install a shareware utility such as Macaroni, JAW PseudoAnacron, or Anacron that will automate the maintenance activity regardless of whether the computer is turned off or asleep.  Dependence upon third-party utilities to run the periodic maintenance scripts was significantly reduced after Tiger.  (These utilities have limited or no functionality with Snow Leopard, Lion, or Mountain Lion and should not be installed.)

Malware Protection

If you are having an immediate problem with ads popping up see The Safe Mac » Adware Removal Guide and AdwareMedic.

Open Safari, select Preferences from the Safari menu. Click on Extensions icon in the toolbar. Disable all Extensions. If this stops your problem, then re-enable them one by one until the problem returns. Now remove that extension as it is causing the problem.

An excellent link to read is Tom Reed's Mac Malware Guide.

Also, visit The XLab FAQs and read Detecting and avoiding malware and spyware.

See these Apple articles:

  Mac OS X Snow Leopard and malware detection

  OS X Lion- Protect your Mac from malware

  OS X Mountain Lion- Protect your Mac from malware

  OS X Mavericks- Protect your Mac from malware

  About file quarantine in OS X

If you require anti-virus protection Thomas Reed recommends using ClamXAV. (Thank you to Thomas Reed for this recommendation.)

From user Joe Bailey comes this equally useful advice:

The facts are:

1. There is no anti-malware software that can detect 100% of the malware out there.

2. There is no anti-malware that can detect everything targeting the Mac.

3. The very best way to prevent the most attacks is for you as the user to be aware that

     the most successful malware attacks rely on very sophisticated social engineering

     techniques preying on human avarice, ****, and fear.

4. Internet popups saying the FBI, NSA, Microsoft, your ISP has detected malware on

    your computer is intended to entice you to install their malware thinking it is a

    protection against malware.

5. Some of the anti-malware products on the market are worse than the malware

    from which they purport to protect you.

6. Be cautious where you go on the internet.

7. Only download anything from sites you know are safe.

8. Avoid links you receive in email, always be suspicious even if you get something

    you think is from a friend, but you were not expecting.

9. If there is any question in your mind, then assume it is malware.

Installing System Updates or Upgrades

Repair the hard drive and permissions beforehand.

Update your backups in case an update goes bad.

Backup and Restore

Having a backup and restore strategy is one of the most important things you can do to maintain your computer. Get an external Firewire drive at least equal in size to the internal hard drive and make (and maintain) a bootable clone/backup. You can make a bootable clone using the Restore option of Disk Utility. You can also make and maintain clones with good backup software. You can never have too many backups. Don't rely on just one. Make several using different backup utilities. My personal recommendations are (order is not significant):

     1. Carbon Copy Cloner

     2. Get Backup

     3. Deja Vu

     4. SuperDuper!

     5. Synk Pro

     6. Tri-Backup

Visit The XLab FAQs and read the FAQs on maintenance and backup and restore.

Always have a current backup before performing any system updates or upgrades.

Final Suggestions

Be sure you have an adequate amount of RAM installed for the number of applications you run concurrently. Be sure you leave a minimum of 10% of the hard drive's capacity or 20 GBs, whichever is greater, as free space. Avoid installing utilities that rely on Haxies, SIMBL, or that alter the OS appearance, add features you will rarely if ever need, etc. The more extras you install the greater the probability of having problems. If you install software be sure you know how to uninstall it. Avoid installing multiple new software at the same time. Install one at a time and use it for a while to be sure it's compatible.

Referenced software can be found at CNet Downloads or MacUpdate.

Most if not all maintenance is for troubleshooting problems. If your computer is running OK, then there isn't really a thing you need to do except repair the hard drive and permissions before installing any new system updates.

Apps like MacKeeper or any other maintenance apps like CleanMyMac 1 or 2, TuneUpMyMac, MacCleanse or anything like these apps, installed on your Mac, while they appear to be helpful, can do too good a job of data "cleanup" causing the potential to do serious data corruption or data deletion and render a perfectly running OS completely dead and useless leaving you with a frozen, non-functional Mac.

Plus, these type of apps aren't really necessary OR needed. They really aren't.

There are manual methods to clear off unnecessary data off of your Mac that are safer and you have complete control over your Mac and not just leave a piece of auto cleaning software in charge of clearing off data off of your Mac. Their potential of causing OS X issues outweighs the implied good and benefits these types of hard drive or memory "cleaning" apps are written to do.

These types of  system 'cleaning" apps are very poorly written and are really a scam to rob newbie and novice Mac users of their hard earned cash for a poorly written maintenance program that will do much more harm to a perfectly normal running OS X system than the good that the app developers purport these types of apps will do.

Plus, the software companies that write these apps make it hard to easily uninstall these apps if something DOES go wrong and these apps work in a way where you have no recovery or revert function to return your Mac back to its former, working state in the event something does go wrong.

It is best to never, EVER download and install these types of apps.

The risk to your system and important data is too great a risk!

How to maintain a Mac

1. Make two or more backups of all your files, keeping at least one off site at all times in case of disaster. One backup is not enough to be safe. Don’t back up your backups; all should be made directly from the original data. Don’t rely completely on any single backup method, such as Time Machine. If you get an indication that a backup has failed, don't ignore it.

2. Keep your software up to date. In the App Store or Software Update preference pane (depending on the OS version), you can configure automatic notifications of updates to OS X and other Mac App Store products. Some third-party applications from other sources have a similar feature, if you don’t mind letting them phone home. Otherwise you have to check yourself on a regular basis.

Keeping up to date is especially important for complex software that modifies the operating system, such as device drivers. Don't install such modifications unless they're absolutely necessary. Remove them when they are no longer needed. Before installing any Apple update, you must check that all system modifications that you use are compatible. Incompatibility with third-party software is by far the most common cause of difficulties with system updates.

3. Don't install crapware, such as “themes,” "haxies," “add-ons,” “toolbars,” “enhancers," “optimizers,” “accelerators,” "boosters," “extenders,” “cleaners,” "doctors," "tune-ups," “defragmenters,” “firewalls,” "barriers," “guardians,” “defenders,” “protectors,” most “plugins,” commercial "virus scanners,” "disk tools," or "utilities." With very few exceptions, such stuff is useless or worse than useless. Above all, avoid any software that purports to change the look and feel of the user interface.

It's not much of an exaggeration to say that the whole "utility" software industry for the Mac is a fraud on consumers. The most extreme examples are the "CleanMyMac," "TuneUpMyMac," and “MacKeeper” scams, but there are many others.

As a rule, you should avoid software that changes the way other software works. Plugins for Photoshop and similar programs are an obvious exception to this rule. Safari extensions, and perhaps the equivalent for other web browsers, are a partial exception. Most are safe, and they're easy to get rid of if they don't work. Some may cause the browser to crash or otherwise malfunction. Some are malicious. Use with caution, and install only well-known extensions from relatively trustworthy sources, such as the Safari Extensions Gallery.

Only install software that is useful to you, not (as you imagine) to the computer. For example, a word processor is useful for writing. A video editor is useful for making movies. A game is useful for fun. But a "cache cleaner" isn't useful for anything. Cleaning caches is not an end in itself.

Never install any third-party software unless you know how to uninstall it. Otherwise you may create problems that are very hard to solve. Do not rely on "utilities" such as "AppCleaner" and the like that purport to remove software.

4. Don't install bad, conflicting, or unnecessary fonts. Whenever you install new fonts, use the validation feature of the built-in Font Book application to make sure the fonts aren't defective and don't conflict with each other or with others that you already have. See the built-in help and this support article for instructions. Deactivate or remove fonts that you don't really need to speed up application launching.

5. Avoid malware. Malware is malicious software that circulates on the Internet. This kind of attack on OS X was once so rare that it was hardly a concern, but malware is now increasingly common, and increasingly dangerous.

There is some built-in protection against malware, but you can’t rely on it—the attackers are always at least one day ahead of the defense. You can’t rely on third-party protection either. What you can rely on is common-sense awareness—not paranoia, which only makes you more vulnerable.

Never install software from an untrustworthy or unknown source. If in doubt, do some research. Any website that prompts you to install a “codec” or “plugin” that comes from the same site, or an unknown site, is untrustworthy. Software with a corporate brand, such as Adobe Flash Player, must come directly from the developer's website. No intermediary is acceptable, and don’t trust links unless you know how to parse them. Any file that is automatically downloaded from the web, without your having requested it, should go straight into the Trash. A web page that tells you that your computer has a “virus,” or that anything else is wrong with it, is a scam.

In OS X 10.7.5 or later, downloaded applications and Installer packages that have not been digitally signed by a developer registered with Apple are blocked from loading by default. The block can be overridden, but think carefully before you do so.

Because of recurring security issues in Java, it’s best to disable it in your web browsers, if it’s installed. Few websites have Java content nowadays, so you won’t be missing much. This action is mandatory if you’re running any version of OS X older than 10.6.8 with the latest Java update. Note: Java has nothing to do with JavaScript, despite the similar names. Don't install Java unless you're sure you need it. Most people don't.

6. Don't fill up your disk/SSD. A common mistake is adding more and more large files to your home folder until you start to get warnings that you're out of space, which may be followed in short order by a startup failure. This is more prone to happen on the newer Macs that come with an internal SSD instead of the traditional hard drive. The drive can be very nearly full before you become aware of the problem.

While it's not true that you should or must keep any particular percentage of space free, you should monitor your storage use and make sure you're not in immediate danger of using it up. According to Apple documentation, you need at least 9 GB of free space on the startup volume for normal operation.

If storage space is running low, use a tool such as OmniDiskSweeper to explore the volume and find out what's taking up the most space. Move seldom-used large files to secondary storage.

7. Relax, don’t do it. Besides the above, no routine maintenance is necessary or beneficial for the vast majority of users; specifically not “cleaning caches,” “zapping the PRAM,” "resetting the SMC," “rebuilding the directory,” "defragmenting the drive," “running periodic scripts,” “dumping logs,” "deleting temp files," “scanning for viruses,” "purging memory," "checking for bad blocks," "testing the hardware," or “repairing permissions.” Such measures are either completely pointless or are useful only for solving problems, not for prevention.

To use a Mac effectively, you have to free yourself from the Windows mindset that every computer needs regular downtime maintenance such as "defragging" and "registry cleaning." Those concepts do not apply to the Mac platform.

A well-designed computing device is not something you should have to think about much. It should be an almost transparent medium through which you communicate, work, and play. If you want a machine that needs a lot of attention, use a PC.

The very height of futility is running an expensive third-party application called “Disk Warrior” when nothing is wrong, or even when something is wrong and you have backups, which you must have. Disk Warrior is a data-salvage tool, not a maintenance tool, and you will never need it if your backups are adequate. Don’t waste money on it or anything like it.