Your confusing hard drive storage space with RAM (Random Access Memory)?
Click once on the icon of your main internal hard drive and use the key command Command-I and tell us how much hard drive specs is left on your iMac.
Before arbitrarily deleting items, you have the option of either burning your data to discs, Flash drives or purchasing an external hard drive and copying or cloning your important data there.
I would save the data rather than deleting it. You never know when you might have a need to recover old data files.
Here are some of my tips for deleting or archiving data off of your internal hard drive.
You'll need to free up lots of hard drive space.
At a minimum, at least, 15 to 20 GBs of space.
Perhaps even more.
Hard drive full or near full:
Have you emptied your iMac's Trash icon in the Dock?
If you use iPhoto, iPhoto has its own trash that needs to be emptied, also.
If you use Apple Mail app, Apple Mail also has its own trash area that needs to be emptied, too!
Other things you can do to gain space.
Delete any old or no longer needed emails and/or archive older emails you want to save.
Look through your Documents folder and delete any type of old useless type files like "Read Me" type files.
Again, archive or delete any old documents you no longer use or immediately need.
Uninstall apps that you no longer use. If the app has a dedicated uninstaller, use it to completely uninstall the app. If the app has no uninstaller, then just drag it to the OS X Trash icon and empty the Trash.
Also, if you save old downloaded .dmg application installer files, you can either archive and delete these or just delete the ones you think you'll never install, again.
If you can still find this app, download and install Monolingual.
This app will delete all other OS X languages that you do not need or use on your iMac.
You can check the languages that you want Monolingual to delete and uncheck others you wish to keep.
Download an app called OnyX for your version of OS X.
When you install and launch it, let it do its thing initially, then go to the cleaning and maintenance tabs and run all of the processes in the tabs. Let OnyX clean out all web browser cache files, web browser histories, system cache files, delete old error log files.
Typically, iTunes and iPhoto libraries are the biggest users of HD space.
If you have any other large folders of personal data or projects, these should be thinned out, moved, also, to the external hard drive and then either archived to disc and/or deleted off your internal hard drive.
You may have to Purchase an external FireWire or Thunderbolt hard drive to move these files/data off of your internal drive to the external hard drive and deleted off of the internal hard drive.
Good Luck!