practice on proportions and shading

i've been getting a steady incline on criticism and i've been trying to fix my art accordingly, but it's been getting harder. this was just a practice picture, but i'm feeling like i'm not going anywhere. if i am, or you know a way to get me on the right track, i would be more than happy to listen. all i ask is that you are specific, and tell me how to fix it. don't just tell me i got something wrong and then leave. i want to improve. :D