Alright, I'm not an expert like the rest of these guys but I'll see what I can do.
First, like all shading, define your light source. It looks like this one is to the right and a bit down? That's a strange choice, but it could work. But, the top of the lower section has some of the darkest shade outlining it, while the top is all midtones with almost no shading at all. Consistency is very important, and you should probably consider an upper light source.
Now, the amount of colors in that sprite is ridiculous. I don't know if you used some kind of tool to antialias it or something, but I came across over 20 colors looking through it, and I only looked closely at part of the bottom.
For pixel art, you want to limit your palette, probably no more than 8 colors for a sprite this size. And right now, don't get into antialiasing, that's something you need quite a bit of experience for.
Pay close attention to the form of the object when you shade. You have huge fields of the lightest color that have no texture or detail, and the rest of the shading is flat and simple. Also, following the outline is not following the form. It creates banding and pillow shading, and instead of explaining that here I'll segway into my recommendation for reading
this tutorial. It's one of the best I've seen and covers pretty much covers all the basics. It does still go over some more advanced techniques, so just ignore those for now.
And two more things. This is more my experience, but rocks probably aren't the best thing to start with. They have very complex shading and textures which I still even have trouble with after years of practice. And finally, practice! That's always the most important. You're at a very novice stage and you can only improve. Look around these forums and see how the professionals do it. I learned the most from studying one of my internet friends' work.
Actually, one more thing. If you're painting inclined, look at the end of the second section of that tutorial. That might be more helpful with getting used to shading forms.
I'd make an edit to show some of this, but I'm on a laptop with no mouse access, so... someone else can cover that, maybe?