If I understand correctly you want to do some java program for cell phone. You need a J2ME development environment so that you may develop some java application, resulted in a file.jar (an executable jar file) ready for cell phone.
The very first thing you need is the the Micro Edition Software Development Kit 3.0
and you need to learn the java microedition...
now for creating a java based mobile based chat app ... you need to specify that what kind of the medium you are going to use.If you are using gprs services then go for the instant masseging like apps or if you are using some another medium for communicating mobile phones .... then you do have the knowledge of that particular tech.... for more help.. drop your query at <EMAIL SNIPPED>
by peter_budo: Keep It Organized - Do not ask anyone (member or moderator) for help by email or PM