I come from the ancient era of "we had to code all by ourselves" and that is what I still do. So, I do not know these new "next-generation" 3D engines/environments where you just apply a bit of code here and there and it miraculously turn into a game. (no, its not really like that, just the way I see them)
That is why I am offering my opinionated answer for you to learn java (as that is your choice, it could be any language) by How to;
- Make simple application
- Draw a shape (circle as an example)
- Another shape attached to mouse location (learn about mouse)
- Check if these two shapes collide (how collision of circle and square would be different?)
A wild'ish example, which I would suggest to do. From there you can use those learned elements to make simple games like so many mouse clicking games. Learn how to measure time and you can make such game more complex by making player compete against his own reaction times, etc... let your mind think of more things and you stumble on more to learn.
While you do simple thing(s) like this, you are learning all the time and most importantly learning programming language of your choice (java for you).
While you learn more about java, nothing stops you from playing around with jMonkey also. In case you feel stuck in jMonkey, go program simple things in java and feel success, if you get stuck with simple java programming, search stackoverflow... I know from experience that SO has huge coverage in answers to java game programming.
See, when its not about jMonkey, when its about general java game programming, there is way more answers, so many good tricks and tips to learn.