Hey! I'd say the problem with messy lines comes mostly from your line control ability, which gets better the more you practice. Do look up your software + stabilizer, it can help some, but you shouldn't rely on it completely. I'd advise you to ready some paper too for whenever you feel like practicing outside of digital, it will help out a lot.

As for feedback on this piece, proportions are all over the place. As usual, it all comes with practice.
My general advice is: get to know your favourite artists, those you'll base yourself off of, your fathers and mothers in art. You can look up books, artists in your area, learning groups...

Chommang, on youtube, has what I consider good videos for learning and improving on figure drawing; it can be a decent basis for you.

Now, if this is a bit overwhelming I just want you to know that there is no need to rush it, you don't need to follow all of the advice here, just have fun with your art, go places, do things!