Hello! I would suggest doing some warm ups doing various different lines (long,short, thin, thick etc.) that will definitely improve your line work over time. I would also suggest to go over the fundamentals in order to understand the structure and proportions of the head. Other than that it is a lovely piece!

Hey Vivian, I'm not very familiar with Manga styles of drawing, and I know somethings are very stylized. This drawing does have a lot of personality to it which is great. I think though that you could improve your drawing by applying some structure. I did a drawing over your head here with some suggestions on how you can make your drawings a little more consistent. I hope you dont mind, and that you get some value from it. Keep drawing :)