Being that you're rather young, you should consider on just slowing down on the whole dating thing. If you want to fix your uneasiness towards girls, you should get some friends that are girls that you are comfortable with talking to. It's best if these girls don't really care about dating. That way, you can get used to talking to girls. Now, I'm not saying that you should start hunting down girls though.
With the girls that you do find attractive, get into some contact with them rather than hiding behind bushes when you see them. If you have an opportunity to talk to them, ask them a casual question. For example, you can just leisurely ask what your cafeteria is serving for lunch today. In doing this, you'll ease into talking to them and you'll make your presence known without having to do something crazy. What you shouldn't do is forcefully approach them and ask them an odd question out of the blue. That'll just weird them out. When speaking to them, relax. You should remember that they're human too. (Also, don't mumble.)
Believe me on these tips. I've been in the dating scene and I know how teenage girls think. I'm a teenage girl myself. You should know that not all people are ready for going out with the opposite gender when they're your age. I wish you luck though.