Well it's actually pretty nice for a beginner. You'd just have to work on the details a little.
For example, you could try to blend it a little better. That's quite easy to do, by either making some more layers over the actual render with brushing on them to make its borders look a little less sharp or you could use the blur tool to make them a little softer. Burning or dodging also helps sometimes. Little things like those make a big difference. Then just work on your border a bit ( black is just a little comon ) and there you go. :]