Q: Old iMac- what to do?

Some more info would be nice?

How old is old?

What is the year and screen size of this iMac?

Depending on the answer, you maybe able, to at least, upgrade to OS X 10.6 Snow Leopard, from OS X 10.6 Snow Leopard, possibly move on to OS X 10.7 Lion OR possibly to the OS X 10.8 Mountain Lion OR upgrade your iMac to the free OS X 10.11 El Capitán.

What iMac do you have?
To find out info about your system,
Click on the Apple symbol in the upper left of the OS X main menu bar. A drop down menu appears.
Click About this Mac. A smaller popup window appears. This gives you basic info like what version of OS X your iMac is running, the speed of your iMac's CPU and how much RAM is installed.
Click on the button that says More Info.
A larger window appears giving you a complete overview of your iMac's hardware specs.
Highlight all of this info and copy/paste all of this into another reply to this post, editing out your iMac's serial number before actually posting the reply.
This will tell us everything about your iMac so we may begin to help with your iMac issues.

Additional info for upgrading to a newer OS X version.

Before embarking on a major OS upgrade, it would be wise, advisable and very prudent if you have a good,working backup of your current system to an external connected and Mac formatted Flash drive OR externally connected USB, Thunderbolt or FireWire 800, Mac formatted hard drive. Then, use either OS X Time Machine app to backup your entire system to the external drive OR purchase, install and use a data cloning app, like CarbonCopyCloner or SuperDuper, to make an exact and bootable copy (clone) of your entire Mac's internal hard drive. This step is really needed in case something goes wrong with the install of the new OS or you simply do not like the new OS, you have a very easy way/procedure to return your Mac to its former working state.

Then, determine if your Mac meets ALL minimum system install requirements.

Mac OS X v10.6 Snow Leopard system requirements

Purchased Installer disc here.

http://store.apple.com/us/product/MC573Z/A/mac-os-x-106-snow-leopard

To install Snow Leopard for the first time, you must have a Mac with:

An Intel processor

An internal or external DVD drive, or DVD or CD Sharing

At least 1 GB of RAM (additional RAM is recommended)

A built-in display or a display connected to an Apple-supplied video card supported by your computer

At least 5 GB of disk space available, or 7 GB of disk space if you install the developer tools.

Then use the OS X Software Update feature of OS X 19.6.3, located in the System Preferences Panel to update your iMac to OS X 10.6.8 and to updated all other installed Apple software and security updates.

OR

Use this download link to OS X 10.6.8 Combo Update installer package, here.

https://support.apple.com/kb/DL1399?viewlocale=en_US&locale=en_US

You now have access to the Mac App Store and you can now continue to upgrade to the next available  OS X versions or skip right to the newest, current, free OS X, macOS system version from OS X 10.6.8.

Depending on your iMac's year, hardware and RAM installed.

Option 1

OS X 10.7 Lion system requirements

Purchased emailed download code here.

http://store.apple.com/us/product/D6106Z/A/os-x-lion

Apple will send you an email for the special download code for the Mac App Store within three days of purchasing the download code.

To use OS X 10.7 Lion, make sure your computer has the following:

An Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor

Mac OS X v10.6.6 or later to install via the Mac App Store (v10.6.8 recommended)

7 GB of available disk space

2 GB of RAM (I recommend at least 6 GBs of RAM)

Option 2

To download and install either the pay OS X 10.8.5 Mountain Lion or the free OS X 10.11.5 El Capitán,  you need one of these Macs:

OS X 10.8 Mountain Lion purchased emailed download code here.

http://store.apple.com/us/product/D6377Z/A/os-x-mountain-lion

Apple will send you an email for the special download code for the Mac App Store, for OS X 10.8.5 Mountain Lion within three days of purchasing the download code.

iMac (Mid-2007 or later)

MacBook (13-inch Aluminum, Late 2008), (13-inch, Early 2009 or later)

MacBook Pro (13-inch, Mid-2009 or later),

MacBook Pro (15-inch or 17-inch, Mid/Late 2007 or later)

MacBook Air (Late 2008 or later)

Mac mini (Early 2009 or later)

Mac Pro (Early 2008 or later)

Xserve (Early 2009)

Your Mac also needs:

OS X Mountain Lion, Lion, or Snow Leopard v10.6.8 already installed

2 GB or more of memory (I strongly advise, at least, 6 GBs of RAM or more)

8 GB or more of available space

Next,

If you run any older Mac software from the earlier PowerPC Macs, then none of this software will work with the newer OS X versions (10.7 and onward). OS X Snow Leopard had a magical and invisible PowerPC emulation application, called Rosetta, that worked seamlessly in the background that still allowed older PowerPC coded software to still operate in a Intel CPU Mac.

The use of Rosetta ended with OS X Snow Leopard as the Rosetta application was licensed to Apple, from a software company called Transitive, which got bought out, I believe, by IBM and Appe  could no longer secure their rights to continue to use Rosetta in later versions of OS X.

So, you would need to check to see if you have software on your Mac that maybe older than, say, 2006 or older.

Also, check for app compatibilty  here.

http://roaringapps.com/

If you have any commercial antivirus installed and/or hard drive cleaning apps installed on your Mac, like MacKeeper, CleanMyMac, TuneUpMyMac, MacCleanse, etc. now would be a good time to completely uninstall these apps by doing a Google search to learn how to properly uninstall these types of apps.

These types of apps will only cause your Mac issues later after the install of the new OS X version and you will have to completely uninstall these types of apps later.

Once you have determined all of this, you should be able to find the latest versions of OS X by clicking on the Mac App Store icon in the OS X Dock and then login to the Mac App Store using your Apple ID and password and if you purchased a download code, input that code.

You can then begin the download and installation process of installing the newer versions of OS X from the Mac App Store.

Good Luck!