Hey, overall is a nice picture. It could seem plain but that's normal because she hasn't colour and shadows, isn't it?
You may want to ignore this as you didn't asked for it and really the shadow and colour is something that matters here.
Let me try to point you some things I would change. But feel free to ignore them. I'm in really no position of telling you anything about drawing.
If the dress is made with fabric then I think it needs to crease more or it looks too plain (probably you'll do that with some shadow). But if the material is like plastic (kinda like one of those futuristics suits of the old movies) then it's ok but remember to give that characteristic "shining" when colouring it.
Is the dress is like this
-shape, not colour, of course-? If it is then I think the piano board around her chest needs some more volumne to define better the separation between dress and skin. But maybe isn't need after you colour it, of course. Just a bit of shadow may be ok.
I like her hairstyle around her head. Good job there.
Maybe the bangs should be a little more "messy". They're too parallel and it makes it look like if they're wet after a sudden rain. I hope you understand what I mean, uh. I'm sorry I can't help you more precisely.
Hmm, I'm not sure what to say about the ponytail. I think it doesn't match the way the short hair is done... hmm, how to tell it... it's like if it were a different kind of hair.
Maybe the shoes need some details but nothing fancy, I guess.
Well, I think that's all the points I can think of about the drawing.