Yellow Dog Linux (YDL) is a PPC flavor of Red Hat Linux. Mandrake, Debian, and (I think, still) SuSE all have PPC distributions of Linux available also.
I just installed YDL on my iBook about a week ago, so I'll give a synopsis of what I did:
You'll need three partitions...
Linux root (or "/"); The main system gets installed here, so it will be about 2GB, or larger.
Linux Swap, which is about 2x the amount of physical RAM your system has, up to 256MB. If you have a "New World" Mac, which it sounds like you have, then you'll need a
Boot partition also. The bootloader (YaBoot) gets installed here, and is about 10MB. When you install YaBoot, it will ask you what other operating systems you have installed on your machine and then configure itself accordingly.
Yellow Dog has a very nice install routine; even allowing you to go a few steps back if you have to correct/fix something. Get yourself an install CD and boot your machine with it... The install routine should start up from there.
Read as much as you can about Linux. Start with something specific that you want to do, and branch out from there

. Learn as much about your machine as you can also... This will only help you to understand what's going on with Linux. Good luck!