If you don't have the OS X disc, then the second reply stands; use the original disc (the Software Install I think - it's three years since I parted with my G3s). You'll know when you boot from the disc anyway - the right one will have Disc First Aid accessable from the menu bar (under Tools I think, but if not just check the other menus).
There is one other way if you're not happy booting from the disc, though.
As it has OS X, if you log in to the new admin account and go to System Preferences > Accounts and click the lock and enter your Admin password (the one for the account you're logged into, not the original one) to unlock the prefs, highlight the original account and click the minus sign below the pane. You'll be given choices about saving the account data - just select delete now.
When that's done, the only account is the one you're logged in to and that has no personal data. To ensure the data you've just deleted can't be recovered, open Disk Utility from the Applications > Utilities folder.
Select Macintosh HD in the side panel, click the erase tab and then go to the button near the bottom which says "erase free space". Click that and it will perform the zeroing operation to securely erase all the areas of the drive where the deleted data resides.
Once again, this does take some time.