I just wanted to say that for a first attempt, this is really good!
The most obvious thing I notice about the drawing is that it uses copy + paste + flip. Pixels on the right are almost all the same as those on the left. That includes the shading.
I would suggest trying this:
Pick a direction for your light-source, then shade everything so it works with the light coming from that direction.
If that is hard to understand with Google Translate, let me know.