It's pretty good for a first attempt.  Good job on keeping the color count down; that's usually a big problem for new pixel artists (I know I went through a phase of using about a hundred more colors than I needed).

I would suggest that you stay away from dithering until you better understand it.  For a piece this small, the dithering more adds texture than anything.  I think for this piece, some simple cel shading would have been the best route.  Keep working on it, though.  You'll get better!