Hey there
I'm not a professional by any means, in fact i'm just starting out but i hope i can somehow help.
Well, the first thing i noticed was the hand, it's too big and it doesn't look like it has bones.
Same with the other arm, but this one it's too small, you need to work in proportions.
Secondly, the hair. It needs to have flow, you need to feel it falling. Feel free to look for reference to know how it moves.
And lastly, the tones. It looks like you just used: the color and then black for the shadows (even if that's not what you did) you should have more variation for it to be more interesting and realistic.
And that's all, feel free to correct me if something i said it's wrong or something .