Suppliments aren't going to be of much value. You are 15 and should be able to eat right, exercise, and get the results you want within a reasonable amount of time. I don't think 3 weeks is enough lead time to do a lot though. Fitness is a lifelong commitment, not a short term challenge.
No better time to start than now though. If you are interested in creating an effective nutrition and workout plan first read the stickies in the beginners section, then in the
nutrition section.
If you have specific questions please feel free to ask. The folks here will be very happy to help you. Feel free to join in the discussions.