how can i change the old owners keychain password into mine

You can't change the password unless you know it.

If you know it, you can change it using Keychain Access.

I don't know it so what do I do

Create a new keychain.

I'd generally suggest acquiring and wiping and installing the most recent version of OS X onto the system that it'll support, as you'll spend more time and effort undoing what was done before. You can usually get the proper distro with a call to Apple.

Given the iBook is PPC, you'll be limited to Leopard (10.5) at most, and possibly not that version.