On first glance, it does look really good, but the shading seems.. off center?? When you shade, you have to keep in mind a light source. Here's a pretty thorough tutorial.

I'm not great at shading, either, so I feel that. I do really like what you've done so far though, keep at it!!

Thanks for the input! Shadowing has always been a constant trial and error thing for myself. I'm always afraid to make the shaded areas too big/small, or the angles are off, or I missed a spot and I never know where to put the light source...ah, yeah. xD I took a quick glance at the tutorial and will definitely go through it more closely when I go to fix my shadows! It seems like a great guide!

While I agree with the comment about shading, I'd like to add: it's very mono-cromatic. With the exception of the mouth and the (black) shading, you're working in one color. Check out your color wheel and when you shade, don't shade in black, but shade in a different color. It adds depth to the piece.

For example, look down at your shirt. Hopefully you're wearing one. XD

See the folds in your shirt? See the way light is moving over those folds? Look at the color. It isn't black (unless you're wearing a black shirt, and then this whole thing doesn't work--go change your shirt). The point is, the color of the dark creases in your shirt isn't black. It's just a darker version of the color you're wearing. Depending on your lighting, it might even be a slightly different color all together.

God, I hope that helps, because that was really hard for me to get out. XD

Otherwise, well done. I like the composition as a whole, and your idea is definitely there.