Join a local gym if you can, or use the school gym. I suggest low impact exercises to help lose weight, like a stride machine or elliptical. Then reduce your caloric intake by 500 calories (to start with). Eat good food, no more fried or fast food.
If you can purchase it, or convince your parents you could really use it, a heart rate monitor is invaluable. Talk with your physical education department about what your weight or body fat % goal is, then have them help set up a program to get you there. Use the school facilities to you greatest advantage.
And run distance, maybe 5 miles to build up endurance. Then focus on increasing pace. If you have time do it this way (nicer to the knees), do it, otherwise HIIT works well for the short time situations.
Pushups, situps, flutter kicks, and swim until you need to float on your back. Then repeat.
If you can do 40 pushups, 50 situps, and run 1.5 miles in less than 12 minutes, you'll be golden.