Put in your Installation disks that came with the computer, or a copy of the Operating System you installed.
Put it in the computer and reboot while holding down "C", this will boot from the CD.
Once you get into the CD's small OS, look at the menu bar and find "Utilities", bring that down and choose Disk Utility.
Choose your root disk and click "Erase", before you erase the drive, look through the Security Options and consider Zeroing out the drive (securely erases all data). Make sure to erase it and reformat it as Mac OS Extended (Journaled).
Re-install the OS.