Please remember that I am an extreme newbie at this. No, I am not asking you to sugarcoat your criticisms - I am just saying that I might not understand some of them.
Shading was by far the hardest part for me. I tried to consistently keep my light source above and slightly behind the person, but I'm sure there are discrepancies in places.

Note: Right click and open it in a new tab for the most detail.
A few things to take note of.
-There's a few places that should have light but doesn't. Be consistent with your light source.
-Clean your lines up! They're dirty and jaggedy.(Derek Yu's tutorial goes over that.)
-Your colors need more contrast. Your shadow color is WAY TO LIGHT. Your medium color is almost the same color as your highlight.
-A few anatomical(idk if this is a word) errors but I'm sure someone else could help you with that.
A couple links to some tutorials that would do you great help:
http://www.derekyu.com/?page_id=218 Derek Yu's tutorial. Pretty decent.
http://www.natomic.com/hosted/marks/mpat/ Marks' tutorial