Before you begin...

Before you begin to think to hard about your game, you should first understand at least the concept behind Java and the jMonkey Engine.

Java actually has some very good documentation about where to start located here. These short lessons are very well written and an absolute must for beginning to understand how to make a game.

jMonkey Engine also has some nice tutorials, though these aren't quite as nice as the Java ones, you should at least flip through them and definitely read the Best Practices section.

Those 2 are required reads, even if they do seem difficult to understand. When you start to see the code implemented it starts to make more sense.