The problem with dithering is that you really need to learn where and when it should be used. And when you're just starting out, I can all but guarantee it's the wrong time to use it. Aside from the fundamental issues with incorrect dithering (describing a rough form, describing texture, where the surface is supposed to be smooth, etc), it's really not the best use of your time. Instead of focusing on describing the overall form and volume of an object, you end up pissing about making little dots. When you haven't even nailed the art of describing a form through blocked lighting, all you'll end up with is a bunch of random dots.
On to some of the more fundamental issues of your piece:
1. As I mentioned before, it looks like a stone relief carving. You
could say it's meant to be that flat, but I guarantee you that thing would be uncomfortable as hell to wear. Plus you'd look like an idiot wearing it (it'd be sticking flat to your face, probably hurting your nose, with lots of room between the sides of it and your head).
2. The mouth seems arbitrarily open, though I'll chalk that up to design decision, and attempt to work something plausible.
3. It's
circular! Take a look at your head in the mirror, and you'll notice that heads aren't circular. They're elongated vertically. Some egg-shaped, others more squared, others wider at the bottom than top, etc. But never circular.
Anyways, threw together a really quick edit. It's messy and unrefined, but I've better defined the volume and forms with sloppy, blocked-in lighting than your dithered mess of dots. And
that is one of the key things Adam was getting at. Focus on studying lighting, how form and volume affect light and shadow.
AA where the light is? sounds good i guess.
And for the love of god, don't even
think of AA at this point.
I may sound harsh, but I mean well.
edit: Evidently I also type slowly. >_<
Adam is giving you extremely good advice, and you'd do well to listen, and actually absorb what he's saying. Faceless also brings up a good point about the material, but I'd venture to advise you not to even bother with material properties until you've gotten a good grasp of basic lighting and form.