You should start learning how to draw on paper. As Gil always suggest: the books from Andrew Loomis are always a good place to start. You can find them here:
LINK.
Then when you feel comfortable about your knowledge in anatomics and light you can swith to pixel.
Here's a small edit I make, not very good one, but would help me explain myself:

* You got to much unnecesary aa.
* Palette is a bit desaturated.
* Try recycling colors.
* Try removing outlines.
Of course you can go for a cartoony style and leave the outlines and don't work with too much shadow. But it depends on what you're going for.
Hope it helps
