Design wise... I think the head is a bit big, almost child-like in size, though the character doesn't appear to be an infant. The upper arms are very very small. Also the eyes are kinda asymmetrical; the ears have no defined structure; and the nose looks very "last minute" because there's practically no room for it and it just sits there crookedly. Lastly his body seems to have very little structure to it, making it kind of blob-like. Some more planning, thumbnails, and construction sketches might be helpful in solving design problems before getting started on the actual piece.
Line Technique... Looks a bit haphazard, even for a sketch. Remember, lines indicate curvature, groves, and other attributes in an image, so to place them arbitrarily is a mistake. Even some of the craziest ink art has a definite structure and purpose to the lines. So I'd work on tightening that up. Slow down a bit if you tend to draw fast.
Painting Technique... Same as the linework; scrawly, haphazard. Definitely need to get some structure and purpose behind the strokes, even if you decide to keep the rough look.
Values (shading)... Many of the highlights and shadows are not aligned with the initial line art; again implying a sorta sloppy technique; things being done either way too fast or without much attention. As already mentioned, lines very often represent changes in value, so align your values with your lines. Also, a side light (rim) is usually not enough to really give a sense of volume in a character. Consider using traditional 3-point lighting if you can, or at least have one light slightly off to the side, but way less than what you have now.
Compositionally... I find it a bit awkward that his arm goes out of frame and then comes back in.
Character pose... It would be nice to see his right arm. Why hide it? Also, the thing he's doing with the eyes is almost comical, and probably assigns way too much intelligence to a creature that looks quite primitive. Usually experimenting with different facial expressions will tell you what is best.
Ok, I'm drained. Hope this stuff helps out!
On top of what Alex-86 said I say that the jaw looks odd. Most teeth are set in bone or cartilidge, I don't get that sense from your guy.
ANd as harsh as this sounds I suggest you work on your skills from the ground up. Some anatomy studies would do you alot of good aswell as studies on perspective. I looked at you DA page and for 16 you definietly have potential but the earlier you start really trying to work out the fundamentals, the better you'll be. Start a sketchbook here and post your studies. People will totally help you out.
"This is a paint and pixel-splattered furnace that forges the swords of artistic mastery. This is a place where swarthy and belligerent dwarves drink turpentine mead, berate their apprentices and slap the trade into their skulls. It's where the anvils are made of graphite, the hammers are as true as rectangular marquee selections and the fires burn with the light of a thousand lensflares." --Jason Rainville
Thanks for the critiques alex_86 and wooden mango, you have given me a lot to think about! Clearly, I don't think this is worth the effort making structural changes at this stage, so I will leave the image as it is now.
I hope to start a sketchbook when I finish school at the end of this year to help build up my skills.