HT1553: Mac OS X v10.5, v10.6: How to back up and restore your filesLearn about Mac OS X v10.5, v10.6: How to back up and restore your files
Delete stuff? Safest is to ONLY delete stuff under you home folder. Deleting operating system files without understanding what they are is a sure way to disable your Mac. If I doubt, ask.
OmniDiskSweeper (free) is a very useful utility for finding where all your disk space is being used. Google it to find a copy.
If you continue to use the Mac without resoving this you will probably lose data.
Start by emptying the Trash and Shutting Down.
Backup everything to an external disk.
Remove infrequently used items especially:
Ensure that they are backed up in at least 2 places first - perhaps external disk and DVDs.
Empty the Trash.
Shut Down your Mac about once a day.
Use AppleJack about once a month with the following command:
applejack AUTO shutdown