Kay,
If you want to reformat your Mac hard drive, you are probably going to want to do some Web research on the subject, as it likely varies by operating system and particular Mac (I'm not a Mac expert so I can't be sure. Hopefully one will pop up and answer this question for us both.)
However, if I were you, I would simply download
DBAN for Mac, a free utility that will securely wipe away all of your data -- a smarter option than reformatting anyway.
If you want more information on how disk-wiping software works, check out our article "
Obliterate Hard-Drive Data with Disk-Wiping Software."
Hope that helps get you started.