This being your first post, I'd recommend you to check this thread
out. We all are a lot older than you though.
I'm gonna tell you exactly what I told another poster with a similar thread like yours.
There is nothing you can do about stretch marks, they are basically (battle) scars. Same with the skin, only time will tell (after you reach maintenance and stay there for a good period of time like 1-2 years or more, depending) and genetics is involved too. You cannot exercise the skin to make it shrink, only the muscles underneath which help the appearance somewhat.
Age may be on your side, the earlier you lose weight, the more chance your skin COULD
(being the keyword) bounce back but it will do what it wants to do.
When you get older, you'll realize things like this is trivial and that there are more things to worry about.