Since no one gave you critiques, Ill write some quick stuff? A bit hard to see it between the huge and unnecessary watermark and the fact the photo was taken on an ipod.
First the good stuff, it's very cute and you're pretty young, so for your age you're doing great! I like the outer shell part with the guns! The detail on the gun that you can actually see (watermark is screwing up the other one) is very nice! Well done.
Now to try to help you out....did you use a reference? I'll assume you did.. Looking at pictures of turrets I can tell right away that yours is slightly more round and fat... [link] silly stuff like that. Watch out for it and look at a reference often to avoid it :>
Your shading is nice and smooth! I would work on the shadow a bit more xD But don't worry too much about that for now. Focus more on drawing with references, but not tracing of course!
For the future I would recommend a) using a scanner if you are able to. (On the stuff you want critiqued anyway, it helps others to see all the detail and effort you put in!)
b) Do not watermark it. maybe sign it, or type your username out on the image file itself. (ps no one is going to steal this anyway! c: )
c) Art wise, use more contrast! See the dark line you made down the center there? Use more shades that are that dark in the rest of the picture. Use lots of different shades to make your art more interesting!