Real simple, since the Mac OS X install DVD has everything you need.
So pop that in the drive, boot the installer, and after you select a language go to the Utility menu at the top. Select Disk Utility, select the drive, and then you'll see an "Erase" tab. There is an options button from which you can select one of about 4 options. The one pass erase will probably take a couple of hours depending on the size of the drive, and times will go up from there.
Can anyone advise me on how to erase the hard drive on my iMac? I'm prepping it for sale and need erase the drive this weekend, Monday the latest. With my PC I simply downloaded a program that worked great.