Securely erase all user data on an old iBook
You can use the Disk Utility program supplied by Apple - and you can tell it how many overwrites you wish it to do, the last time I read that info it said one pass was not very reliable, 7 should be fine and 35 was overkill - bear in mind how long it takes to do one complete pass....

A similar question is here : Selling a Macbook Pro - removing personal information from it

Edit: You need to boot from a different media as it cannot work on itself so, either the original install disc or a newer install disk for a later version - as older ones don’t usually work forwards... Other bootable media may also work - but that I have not tried...