Well, I must say for a first attempt, great work!
There are a few things however that can be adjusted...
With green screens as well as blue screens there is a problem with compositing. What happens is that objects and actors sometimes get this black outline (with blue screens) or a green outline (with green screens). To defeat this problem you may want to 'feather' your edges to make your actors blend more into the background instead of looking as if they were 'cut out'. Also another technique to try is to have something called IBL (Image Based Lighting) what this refers to is if your scenery has a brown or blue tint, or any other color, you may want your actors to display a similar tint as well.
Hopefully this was to some help! Good luck on your green screening work, its an excellent attempt!
Imagination and discipline are like yin and yang; both are completely different and yet without each other they are nothing.