Reformatting a new used iMac

Recently went all out purchased a mbp and love it have no problems at all. Like it so much I bought a used imac g5 for home. Problem s the preivious owner had purchaased used and I have no start up disks. How do I reformat and install new Osx like leopard or even reinstall what is already running 10.4.9? And is it possible to save some apps I'd like to keep. I know this is getting long already but te original owner partioned the hard drive into one of 75 gig and the remainder in the other does this affect reformatting? Lots of questions thanks all I know your up to it.

You can either buy retail copies of Leopard or Tiger (make sure they are not grey one mac only disks but full retail copies)

you can then do a clean install and archive the old set-up (an option when you install)

If the owner had partitioned the drive - you may need to back up the old system using SuperDuper to another drive - then get back any apps using migration assistant

cheers, louishen much appreciated btw hows my English accent?