You might want to elaborate on your idea. “Galactic Football” can be anything. In general, the tutorials in the wiki (https://jmonkeyengine.github.io/wiki/jme3.html#tutorials-for-beginners) are a good start for learning the engine. Because this is an engine, you can create basically anything, given enough time. And once you’ve got the basics down, people here are really helpful with problems you run into.
My advice is to start small on your idea, so once you’ve worked your way through the tutorials, you can (If that is your idea) start working with physics to get a ball that a physics character controlled by the player can push around. Go from there.