I need to clean the hard drive on my old iMac. How do it do that?

If you mean erase the hard drive and start fresh, It depends on which OS X is installed.

For a Mac an OS X prior to v10.7 Lion, insert the install disc while holding down the C key. From there you should be able to reformat the drive and reinstall OS X.

For a Mac running v10.7 Lion or later, startup the Mac while holding down the Command + R keys. From there you can reformat the disk and restore OS X using OS X Recovery.

If you are just referring to maintenance, your Mac runs maintenance in the background for you.

What do you mean by clean? It could mean you want to wipe it start fresh, or it could mean you want to clean because you feel it needs cleaning. In general a Mac OS X hard drive do not need cleaning.

Please download and run EtreCheck, created by one of own helpers here in ASC. It is a diagnostic tool that's very useful to us in finding problems. Also it will give us further specs on your Mac. After it runs post the log file here. It will contain no personal information

Here are some general tips to keep your Mac's hard drive trim and slim as possible
You should never, EVER let a computer hard drive get completely full, EVER!
With Macs and OS X, you shouldn't let the hard drive get below 15 GBs or less of free data space.
If it does, it's time for some hard drive housecleaning.
Follow some of my tips for cleaning out, deleting and archiving data from your Mac's internal hard drive.
Have you emptied your Mac's Trash icon in the Dock?
If you use iPhoto/Photos or Aperture, both have its own trash that needs to be emptied, also.
If you store images in other locations other than iPhoto/Photos, then you will have to weed through these to determine what to archive and what to delete.
If you are an iMovie/ Final Cut user, both apps have their own individual Trash location that needs to be emptied, too!
If you use Apple Mail app, Apple Mail also has its own trash area that needs to be emptied, too!
Delete any old or no longer needed emails and/or archive to disc, flash drives or external hard drive, older emails you want to save.
Look through your other Mailboxes and other Mail categories to see If there is other mail you can archive and/or delete.
STAY AWAY FROM DELETING ANY FILES FROM OS X SYSTEM FOLDER!
Look through your Documents folder and delete any type of old useless type files like "Read Me" type files.
Again, archive to disc, flash drives, ext. hard drives or delete any old documents you no longer use or immediately need.
Look in your Applications folder, if you have applications you haven't used in a long time, if the app doesn't have a dedicated uninstaller, then you can simply drag it into the OS X Trash icon. IF the application has an uninstaller app, then use it to completely delete the app from your Mac.
To find other large files, download an app called Omni Disk Sweeper.
http://www.omnigroup.com/more
Also, Find Any File
http://apps.tempel.org/FindAnyFile/
Typically, iTunes and iPhoto/Photos libraries are the biggest users of HD space.
move these files/data off of your internal drive to the external hard drive and deleted off of the internal hard drive.
If you have any other large folders of personal data or projects, these should be archived or moved, also, to the optical discs, flash drives or external hard drive and then either archived to disc and/or deleted off your internal hard drive.
Moving iTunes library
http://support.apple.com/kb/HT1449
Moving iPhoto/Photos library
http://support.apple.com/kb/PH2506
Moving iMovie projects folder
http://support.apple.com/kb/ph2289
A disclaimer,
Things to consider before moving your iPhoto/Photos Library Folder to a new or external location like an external hard drive.
If you make movies on any iDevices using iMovie for iOS,, then transfer the video footage, the IOS version of iMovie saves the footage as a movie file in IPhoto/Photos for IOS and will automatically get transferred to iPhoto/Photos for the Mac when you upload the video from your iDevice.
Newer versions of iMovie will work and link those video files found in your iPhoto/Photos Library on your Mac, but those links can be lost if you move your iPhoto/Photos library and you will not be able to relink that video afterwards as the current versions of iMovie seem to not have a relink option for the video portion of the files (ironically, current versions of iMovie HAVE the ability to re-link the audio files from the video footage, though (The inability to re-link the video files could be a possible bug or oversight in current versions iMovie).
The lost video links show up as "blacked-out" video blocks with no content.
Before moving the iPhoto/Photos Library
If you make movies with iMovie using iPad or iPhone video then 'Consolidate' the files before you finish. This will gather (albeit by duplicating) all the relevant files in the project in one place. After consolidating/duplicating all of the audio and video footage to a seperate, independent location,it should be safe to move your iPhoto library.
The potential way to circumvent this issues maybe to try and import iPad and iPhoto/Photos video directly into iMovie which would be another solution.
Good Luck!