Your off to a great start! Your selections are my main observations. My biggest tip is that you should learn to use something other than the "Quick & Fast" selection tools such as the Quick Selection and Magic Wand tool.
Learn to use the Pen Tool! It will be the best thing you ever learn in Photoshop! It will seem difficult at first and then it will hit you! You will say to yourself, "Why didn't I learn this tool sooner!".
As for composition....if your going for realism.........there are few other minor issues. The main one being that the road is not aligned with the cars direction of travel.....
Then there are some lighting differences that are off............the added background would have similar lighting as to what you see on the dash of the car in intensity and direction.
Well done, I do like the 'space man' there were two old ladies in North Wales who saw a couple of silvery figures by the roadside, just reminded me of that.
As soon as I saw it I thought, Thelma and Louise'.
I found that once I'd learned the tools they became a minor concern and my major concern was actually finding stuff and putting it together. What I did was to go around and take pics of anything that caught my eye, even on a walk to the shops, lots of textures at my feet, found an old clock that someone had trodden on last week looked really good.
You've already got the imagination, follow your instincts and do what you like the most.