It's very good, especially as a first sprite!
It's a good still sprite, but the real trick to learn spriting is doing spritesets that can be animated. It's the perfect way to learn what to simplify and how to organize the color areas to make them distinguishable (of course, with pre-existing characters, you have an extra difficulty, as you can't do a Super Mario and add a mustache, some overalls and a hat to avoid any problems you had.
Also, in general, I try to avoid (even if I don't always succeed) "pixel L" in the blach outlines, but sometimes they make the shape read a little differently, so they shouldn't always be avoided.
Also, I don't know Avatar, and I did recognize the character, wich is always good.
(This is what I meant for "L" pixels.
But, of course, what's important is that the sprite looks good, and I think the head is a good example of nice "L" pixels