You must have a pretty much advanced grasp in Java fundamentals first. That would be very essential =)
If you're the advanced java guy, check out jmonkeyengine:
http://www.jmonkeyengine.com/
Its a very sophisticated 3D library for java. Read the API reference documentation thoroughly and find some classes that will benefit you from creating your RTS.
BTW RTS (Real Time Strategy) games are quite difficult to construct, I'm creating one now using Flash CS4, its a command & conquer type game and im about 50% complete, I started creating it april of this year.
Hope it helps!