That's an awfully open ended question! It really depends on what sort of setup you have, what kind of expertise you have with hardware, and a few other things.
I personally would connect an ethernet cable between the two computers (unless they're already on the same network-you can skip this step if they are), then enable file sharing on both computers. Finally, I'd move the MP3s from the PC to the chosen folder on your iBook. A lot of people report having better results when they move things from the PC to the Mac rather than trying to pull them from the PC.
There are probably a hundred other ways, from burning them to CDs to using an iPod as a transport medium. I think what I explained above is the simplest. But that's just me!
