EDIT: This is specifically how to reformat... sorry, the other posts came up while I was writing it.
1. Put the system install / restore CD or DVD that you have in the drive.
2. Launch System Preferences -> Startup Disk. Select the CD / DVD from here, and click the restart button inside the Sys Prefs window.
3. The computer will shut down and then boot off of the install disc.
4. When you go through the prompts, you will get to a window that has an "Options" or similar button, I think, in the lower left corner. Click it.
5. You will get a list of ways to install. One of them will be something similar to "Erase and Install." Follow the directions. You can use any of the methods of erasing. The ones that write zeros to the disk are more secure (prevent you from losing your data), but may take a long, long time.
Let me know if this helps?