Hey mate, I really like the picture so far, I can definitely see the kind of direction you're going with it.

Digital art is hard and you shouldn't be discouraged if your struggling to lay the foundations of a picture in at the start.

You look like you know a lot already but serious advice would be to not use that airbrush, get a square hard edged brush first and block in the shape and shadow of stuff before you try adding details (like leaves) or softening shadows on smooth curves.

This is only a pixel art version I did but I'm hoping it helps you think about where to put some shadows. I hope you like it.



PS. Good luck with the leaves, they're guna be a b*tch.