learn how to use Recovery Mode
Don't do anything until you can restore and have repaired the disk drive.
Post should get moved to either ML (10.8,.4) or iMac forum
Swap out the hdd for an SSD : )
Don't do anything to clean it up unless you don't know how to first remove browser history from within the programs; low on disk space (I consider more than even the 15% minimum, more like 3--40%)
And do you have page outs? more than enough RAM? what actually leads you to ask? Programs aren't opening properly?
Sometimes it is easier to take the long road: just reinstall and restore.
Recovery Mode
http://support.apple.com/kb/HT4718
TimeMachine 101
https://support.apple.com/kb/HT1427
http://www.apple.com/support/timemachine
Mac OS X & Mountain Lion Community
https://discussions.apple.com/community/mac_os
General purpose Mac troubleshooting guide:
Mac OS X: Starting up in Safe Mode - http://docs.info.apple.com/article.html?artnum=107393
What is Safe Boot, Safe Mode? (Mac OS X) - http://support.apple.com/kb/HT1564
Isolating issues in Mac OS X
Creating a temporary user to isolate user-specific problems:
Isolating an issue by using another user account
Identifying resource hogs and other tips:
Using Activity Monitor to read System Memory and determine how much RAM is being used
Starting the computer in "safe mode":
Mac OS X: What is Safe Boot, Safe Mode?
To identify potential hardware problems:
Apple Hardware Test
General Mac maintenance: