To get started, you put the first restore CD or DVD in the drive, select the Startup Disk item from System Preferences, and then select this disc as the boot disc and click the restart button in the window. This is the most trouble-free way to boot off the disc (for some reason, not all macs respond to some of the other methods...my biggest challenge installing Tiger was to get my iBook to boot off the disc!).
When you get into the restore disc, follow through the options until you get to a window that has an options button in the lower left. Click on that, and you'll get the Erase and Install option. You can overwrite the disk one or more times, to "really" erase it, so that data cannot be recovered from it, but be forewarned that this will take a LONG time. A long, long time. Especially seven pass.
Good luck! 🙂