So you want to erase all your data, right? Boot from your install disk (hold down option while booting) and select Disk Utility from top bar and go to Erase tab and erase
1. Insert the Mac OS X CD.
2. Restart the computer.
3. Immediately after the startup sound, press and hold the "C" key to start up from CD.
4. When the Installer screen appears, do not click Continue. Instead, choose Installer > Open Disk Utilities.
5. Select the hard drive to erase.
6. Click the Erase tab.
8. Click OK.
9. Click Erase.