I'm a man, but I believe push-ups can help increase the "look" of your breasts, at least to some degree. You will build up the muscle behind the breasts, so it should lift them forward. You can also target your lower or upper breast by doing incline (hands on a chair), or decline (feet on a chair), for lower and upper respectively. If you go to the gym or have some weights, there are also many other effective exercises for the chest muscles.
Still, I must say that personally I like smaller breasts more than bigger ones. I like the look of them defeating gravity, and pointing out in their own direction, instead of hanging. I think they look more sensitive and arousing.
Obviously also posture has a huge impact on your perceived breast size. Perhaps you already have a strong chest-forward posture, but if you don't then you can also work on that. If you have very small breasts, the best thing is to walk them as you are really proud of them. In my opinion, posture is much more important than most physical details, size and shape of our body parts. Our posture sends out a constant energy which we react on, consciously and unconsciously. And this energy is much more powerful than any centimeters of measurements. This is actually what we mostly react on when we look at eachother, but we don't really think about it too much, or discuss it. Just look at yourself in the mirror, and take a weak posture with chest in and shoulders forward, and compare to the opposite. The change is so huge that you can hardly compare it to any other physical quality.
You are also very young, so your breasts may change also naturally later. Be aware of sending out a strong and proud energy, and you will look great.