Nagging about some proportion and pencil work things would be too much. You starting, learning and that's good. If you're serious you'll pick up things on your own, or someone will teach you. I'll just say that you should spent more time on hair, chin is objective thing (maybe u like girls with strong chin, what do I know... :P), and next time you do face shadows, take the mirror, position yourself close to some lamp and watch the shading on your face, then do the same on your drawing. Nature can't be wrong, and she's great teacher.
One thing that made me is a right eyebrow. Why the hell she has one eyebrow normal, and the other one is like ''Spock from Start Trek'' style?