That's very good for a first pixel!
Two things: The shell has better shading than the head and tail - it doesn't look soft enough, try making it more like the shading on the shell, I think that would be much better. The lower right part of the shell should have a shadow like the you have in the center of the shell.
Keep up the good work 
Very good for your first work ever. Of course, it could be more detailed and some colours look strange, but I still like it.