The G4 iBook doesn't have an audio input port, so you're going to need some sort of external audio interface. There are many different ones available with all sorts of different features. Probably the simplest one is the Griffin iMic:

http://www.griffintechnology.com/products/imic/

Which should work just fine for what you're wanting to do. Then you can use GarageBand to record the line input and then burn it to a CD with iTunes. Other than iTunes, Audacity is a good bet (and best of all, it's free!):

http://audacity.sourceforge.net/