Hello! I also made you an edit.
Some things to remember are that when working at this size, you're implying much much more than defining. The prime example I see in your sprite is the buttons on his shirt. They are unnecessary, and clutter up your image so that the more important things get lost in the details. Just try to imply hands, don't actually draw them.
You seem to be pillow shading—or not defining a light source and making things lighter as they move towards the middle of the section—instead of actually shading. There are really three different "stages" of shading evolution in an artist. The first is what I just talked about, the second is contour shading, where you shade each section by its shape in two dimensions—sometimes disregarding how each part reacts to the other. And then finally form shading where you shade a lot like a sculptor, cutting and curving, trying to dig into the canvas and really bring out the third dimension. Really strive to get to this form shading.
Your character's anatomy seems a little bit disjointed. I'm not sure what say for this other than, look at naked people! Or as little clothing as you can stand. Then look at people with clothes on to see how the fabric works. Please note that my edit is not even close to perfect in this regard.
EDIT: I'd also suggest to lower the value of your skin tones a bit. I didn't play with the colors of your sprite that much, so just keep that in mind.