How do I delete everything off my Mac?
Non-paranoid - boot from installation CD, use Disk Utility to format the disk(s), perhaps using "secure erase" option (let it run overnight, it's tedious on a disk of any size) install fresh copy of the OS from the CDs, sell the computer.

Reasonably Paranoid - remove disks, sell computer without disks. Disassemble disks.  Be sure to save the very nice refrigerator magnets. Scratch, bend, hammer and otherwise destroy the nice mirrors in there (though simply opening them outside a clean room puts the data past most anyone but the NSA - this is the paranoid option, so overdoing works.)