Draw, draw, draw. And when you're done with that draw s'more. Digital art is fine, but you need to know the basics of art first. No computer program can fill in for a lack of knowledge of the fundamentals of drawing and painting. A computer program is just a tool as much as a paintbrush or pencil is a tool. It's only as good as you are. There is a lot of advice I could give you about your piece. And all that advice has to do with artistic technique and not with the program you're using.
If you want you can email me at email@example.com and I can give you a full critique of your piece. As well as show you many of those artistic fundamentals that I'm talking about. We would need to toss visual files back and forth so doing it over deviant art is not the best idea. When you email me, just let me know what it is about and I'll give you a hand.