A swiss ball is a really could investment. You don't need anything else for your core besides a swiss ball... floor works, but the ball is much more effective. If you're using the floor try planks, those are good and there are several varieties.
Just run outside. When you're 15, you have all the time in the world to do random crap. I'm 20 and in college and I still find time to work out everyday and do other constructive stuff. But, I don't have a job (so no paintball).
The more you work out, the MORE energy you will have throughout the day and it will be easier to go to sleep.
Also another good investment is a bike. Whether you want cardio, muscle tone or a six pack, the answer is a bike and a couple miles a day/every other day.