Firstly, make the picture smaller. Then, tell me why her zig-zag hair is in front of her shoulders and turns blue there. After that, explain if those are white highlights on her hair. Also, her clothes/chest, don't make the outlines to thick nor the strokes too thick and maybe add a little shading. Her facial features might want some enlargement, to avoid inferiority complex, hehe. Especially the shrinking eyebrow on the left, or, is her eyebrow that thing connected to the eye? I'm sorry, I'm not very used to those anime eyes, despite seeing them all over the place. You might want to study anatomy for the neck and space under it. Now that I'm writing about that space under the neck, is that supposed to be cleavage? If so, it looks out of place, because, she looks more on the flat-chested side. Does she have cat ears atop her head, because, something's definitely going on there.

I apologize if I sounded harsh. I was trying to help not sound like an artillery battery. However, I understand that this was just thirty minutes' work. Keep it up and Good Luck in the future, man.