Giving you my honest opinion, I think it looks good so far for a start.
Suggestion: Try to add more effects so that you can give the poster a certain atmosphere and mood. Also, if you have the smudge/blur tool, smooth out the edges of your characters. They will look like they're actually in the poster and not cut and pasted on, giving it a more professional touch ^^

If you want to go even further, try giving your poster a focal point. Your focal point doesn't always have to be a person. It can be an animal, object, or even the background!