I'm new in java ..
Do you have experience in programming? For example C#/.NET or C? If so then by all means start doing 3D or use JMonkeyEngine or LibGDX. If not then its really good advice to start with the basics: Java programming with no frills or librarues, for example progress along lines like these:
- an application printing "hello world" to the screen
- a simple text adventure
- tetris using Java2D graphics
- a simple shooter using LibGDX
.. and that would be plenty to keep you busy for the next couple of years (depending on how much time you can spend on it each week).
Anyway, welcome and good luck learning!