RebelAir pretty much hit the nail on the head.
Some things I want to throw out (with respect), the shape of the iris is wider and shorter than it should be. Keep a circle stencil handy at all times. They will help you keep your sanity. The pupil could use rounding. Also, if you are going for a more realistic look, try adding some yellow and pink tones to the white of the eye.
I'm a beginner too. It gets easier. Things I've learned that have helped me the most:
-Work in light coats, strokes or dots.
-ALWAYS clean the tip of the needle when you stop spraying. Even for a second.
-Use a dark brown, blue, green or violet, reduced, to start laying dark areas. Black has a tendency to over do it for me. I only use it (sparingly) at the end for more definition and contrast.
-Don't be afraid to use stencils. You just can't get the same effects without them.
-As a beginner it may be a good idea to draw it up first on paper or a tablet and print it out. Use the original sketch as a stencil for general outlines/mapping. You'll have more time to spend on details this way while keeping the true shape.