Well, the most important thing for someone in your shoes is diet. I don't know what your current diet looks like, but now is the time to commit 100% to cleaning it up. When you're younger, you can get away with a lot more, but at your age, it's not the time to spin your wheels. Beyond the impact on your appearance, it becomes very important for general health.
Regarding exercises, it wouldn't be appropriate to recommend specific modalities without knowing more about you, but for obvious reasons, you'd want to keep things pretty low impact.