CALLING ALL TECHIES ! How do I upgrade the OS on an iBook ?

I just recently got an iBook G3 ( yeah yeah yeah old piece of crap - I know . ) It runs on a OS X 10.3.9 . This system sucks because you pretty much can't do anything with it . Just wanting to know how to upgrade to 10.4 or higher . Do you have to buy it or is there a free version ? ASAP answer if you can . Thank you ! :]