Boot the Mac from your original CDs that came with the machine or Disk 1 of the Jaguar/Panther retail set, which ever you have. Put the disk in, restart and hold down the 'C' button till the machine starts up from the CD.
When in the setup stage, run Disk Utility (in the top menu bar) to erase/format/partition the disk. The Erase option will have the zero option...