Bruce Eckel's "Thinking In Java" isn't bad for beginners. And get yourself one of those "Java In A Nutshell" books, they are really handy when you are serious about some programming.

Next thing you do is you checkout the LimeWire sources, think of a little something you would like to change and try to change it.

If you have any questions, you can ask them on one of the devel mailinglists:
dev@core.limewire.org
dev@gui.limewire.org