there are a few things to look up that'll help you out here. Keep in mind there is no simple "advice" there are no easy fixes.
First, from a layout perspective look up "the rule of thirds", that'll clear up some of the space issues on that first picture.
If you're going to practice anatomy (and you should every fucking day [trust me]) use reference. Get yourself a book on anatomy, or look up a good anatomy website....I'm sure...you know a good anatomy website....
but do yourself a favor and DON'T practice anatomy without looking at reference or you'll just get stuck in bad habits (trust me again).
As far as shading goes, you don't have to just fuck up until its right, you CAN find books about shading. The best way I've found to learn however is still life. set something on your desk with a light on it and draw it. then move the light and draw it again. try to get it as perfect as you can, spend a few hours on it, you'll never ever ever regret hours spend working on your own skillset.
As far as general art goes, I know you're 16 and you don't have a shitload of experience and that's fine but here's a few things that you could work on just in general:
Blocking out your drawing (sketching) before you add final details on it.
Clean line work (especially if you're going to do more cartoon stuff)
Proportions (comes from using reference and drawing forever usually)
Good use of color (don't rely on blacks and greys, in fact dont use them if you can avoid it)
Anyone can draw, all you have to do is want it. Don't get discouraged and keep drawing! You'll get there if you really want to!
And for fucks sake don't every focus on "style"
trying to "find your style" is for jackasses
just keep drawing what you want, (and honestly what you dont want too)
Hang in there