Step into the light, spoonerman!
Ok, couldn't resist, serious now;
Work on anatomy and such. A good understanding of proportions and such will really show. Not an expert myself, but here are some issues: neck and shoulders lack structure, eyebrows missing (intended?), ear in awkward position, other one missing, should be at least a trace since its big. Face lacks some recessed areas in general, see if you can get some reference.
Too white. Pure white is almost never a good idea I think.
I think giving her some more room would also do her well.
Get the general shape down before you go into details. Makes it easier to spot mistakes and correct them early on. Its also easier to create a good image of the proportions and pose.
I'm not a pro, but a mere amateur trying to improve his realistic drawing aswell, hope I wasn't too harsh.
Pretty good try I think, big leap from manga ^^. Keep improving and make sure to post your progress so we can comment on it!
I can't quite see where the chin ends and the neck begins. I realize that it probably would get cleared up after some refining but the jaw line is all sorts of messed up which actually looks like it pushed the lips over the the right a bit. Giving it some more color and contrast would help a lot too, I think. Not only in the skin but overall in the picture.
I agree with Anthis too. Especially his last paragraph.
I agree with Anthis that the neck and shoulders don't have much structure, they look like they're just hanging. Also, her arm is cut at a somewhat awkward angle, it might be nice to extend that area of the frame a bit. Her left cheek looks caved in compared to the right.
I could also say something about the breasts, but that'd be my own prejudices showing In any case, you're off to a good start!