How do I reset my Powerbook to factory settings?

I have a Powerbook G4 17" with 60GB of which about 55GB has been used. I have been removing/archiving images etc to an external drive but it never frees up that much space. I do however have about 24GB of itunes music/video.

Now having bought a new mac I want to wipe the Powerbook clean and install OS 10.5.6 so that my son can use it. It will be more than enough for his needs for now but how do I go about this.

Thanks in advance.