What should I do [ Teen Diet/Exercise question]?

I am 15 years old weigh 180 pounds and am 5'9, and am a guy. The whole past year I have spent in a cast [various broken bones] and could not exercise the whole time, and gained alot of weight. I also started a medication that highly increases my triglycerides, and need to be on a low fat diet. What are some ways for me to lose weight and look better before school starts [September 6th]