Well, making good sprites isn't something you can just "do". It does take a lot of work (I'm not that good at it ), but it also takes practice.
I don't really know what kind of art you're hoping to do (pixel, realistic, cartoon, a combination of several of those, etc.), but each kind is different. In any case, art takes a long time.
My best recommendation is to look up lots of references of what you want to draw- all different types of references, too. The best example I can give you is the process I used to make a fairly realistic wolf sprite over the past few months. Over the course of making it, I looked at tons of pictures, drawings, runcycle animations, etc. Finally I ended up with a base I was proud of.
It also takes a lot of practice, like I said. Don't get discouraged if you can't make something you like right away. I go through "spurts" of inspiration, as well as dry times when nothing seems to look right.
Most important of all, don't give up! You're still young, and sometimes skills like computer drawing take time to develop. My computer art when I was twelve looked like BLAAA! Now it's gotten a lot better because I kept trying and practicing (I'm still a teen, by the way, so I'm not that much older than you ). So keep trying!
Other advice:
Check the tutorials- there are a few on drawing.
Also look up Lostgarden- the person who owns the site made an AMAZING tileset that they're letting anyone use so long as they give credit if they use it. It's professionally done, too!