Well, you'll need a server computer to host the chat room, this
could be your own computer if you want. For the GUI, if you don't
know how to use Swing and AWT, go to tutorgig.com and search. For
the actual chatting part of the code  go to
http://java.sun.com/j2se/1.4.2/docs/api/index.html and then go down
to "java.net" click on that and it will have the documentation. I
haven't ever used Java's sockets myself so I don't have any more
advice beyond that... except if you hit a small snag just ask on
Yahoo! Chat, I don't remember to many people but I know for sure if
someone named RidiculousFish in on there he'll help you... he's one
of the programming god-type people.