Hi.
Nice fanart
I guess that you are fairly new in the digital arena. There's a few things you can start with and if you keep practice, it will help you out a bunch.
First you need to practice your brush control, because your lines are all over the place making it look sloppy.
Light source. Pick one and stay with that. you have light sources from all direction.
Start with cubes and work up to more complex shapes and you should make great progress.
Keep it up