How to improve my drawing?

You could spend your free time drawing/practicing on your artwork in a doodle book or a clipboard full of printer paper. Each picture will be better than the last. You could also watch some anime speed draws to give you some ideas on detail and shading. I did all the following and I'm an anime artist today. Don't give up, you're going to do great! :)

I like your drawing, especially the hair of the girl. :)
My advice to you is that you could try starting off with the basics, like just practicing different eyes, noses, mouths, hairstyles etc.
You could always watch tutorials or borrow books on how to draw body parts in the right proportions and on angles.

practice practice practice :)
try to imitate some anime arts and apply your own style to them ^^

It's perfectly fine as a cartoony-style drawing but if it's a slightly more realistic style, there are a few problems with proportions. The shape of the head looks a little off. I suggest looking up drawing tutorials on the internet about the shape of the head.

The best teacher is always experience. Keep practicing and drawing during your free-time and in no time you'll be a lot better. Don't be discouraged when it seems your newest drawing looks worse than the last. Things like that happen sometimes(based from experience), and it just means that it's not your day.

Happy Drawing! ^^

By the way, I'm Blackrock89 from deviantart. Nice to meet you.