How do I get the most recent version of OS?

I was given a powerbook G4 mac the other day. It is a few years old so it's version is Mac OS X 10.2.8. Of course I wanna get all the new itunes and stuff like that but it says I need 10.4 or something like that. Well I went to apples download page to look for a more recent version. Well if I do a 10.4 one it says I need a previous version of 10.4 to download the most recent one and the same with 10.3 How do I upgrade to (10.4.something) if it won't let me upgrade unless I have 10.4 and the same with 10.3. I would like to upgrade soon so some would be awesome.

If you want to upgrade the OS you will have to purchase the new version. It is not available for download.

or.... you may want to use google to find your answer. the answer is there, but you will have to look a little.