In order for you to reset your iBook and clean it of all personal information, you're going to require the system disk the iBook came with or a retail copy of Mac OS X compatible to your iBook.
Inert the media disk and hold down the 'C' key during start up. Follow the on screen instructions to drive selection. Select option when promoted to select a destination drive. In options, select 'erase and install' Click next to continue erase and install. Follow all further on screen instructions to seeing up new account ect...
This will erase the disk and install a fresh copy of Mac OS X.