Tips, exercises, and advice on how to lose weight over summer?

I am 200 pounds and only 5'4" and female. Summer break just started. I want to lose weight for my senior year. With senior prom, pool parties and a lot of other fun activities coming up I would like to be a healthy weight or as close as I can be to a healthy weight by september. What are some exercises I can do? Any other advice or support would be appreciated. I've been wanting to lose the weight for 2 years now, but I never did. This is my last chance to go back to school a healthy weight, so thank you.