Yes I'd recommend you stay away fom 3D. It requires some advanced math to really understand and work with that I don't think you are likely to have studied yet.
(Its stuff I didn't do til college.)
So lookign at 2D action games there are a number of different kinds. The easiest are like the first video-games (pac man, asteroids, etc). These have a fixed backgrounds and a fairly small muber of things moving at one time. I would suggest you look at some of these "retro" games .
There are lots of web sites that have versions of them. Some of the pulbishers like Atari and Williams have re-released packaged collections youccan play on your desktop. (There's also MAME, but that requries ROM images and, as they are illegal I can't tell you where to get them. Maybe you have a friend who already knows...)
Playing some of those aught to give you some good ideas of what you might want to do.
The next sep is learnign how to do that sort of input and animation in Java. There are a number of good examples that folks around here have written.
(As I recall there's a real nice pac man clone for instance.) I'd ask the folks here to give you pointers to where you can download some of their examples.
Finally, there are some good resources that you may want to look at as you are studying that code. There are two articles in particular that I know of that explain some details of 2D active rendering better then the API docs do.
I wrote one called "Understanding AWT Image Types". It used to be posted here but when we changed the site over it was a casualty of the move. There are copies around on the web, I'lll try to find a copy for you.
The other, on the direct rendering process, was writen by Mike Martak and can be found on the Swing Connection here:http://java.sun.com/docs/books/tutorial/extra/fullscreen/
If you havent already downloaded them you want to go to java.sun.com and download the J2SE API docs as thats the fundemental reference for all Java programmers.