Well, studying anatomy is the place to start. Proportions can be a matter of personal style, but when learning to draw anatomy it helps to work with more realistic proportions at first.
So if you want to improve, a good thing to do is find photos of people and try to copy the poses. You can continue to draw cartoons, but practice by drawing realistically.
Other improvements like cleaning up the lines or adding details aren't as important until you have anatomy down.
I'm curious what's going on in this picture, it's an interesting scene if nothing else.