Most teenagers have a lot of trouble getting the motivation to get out and do some exercise. And with homework and friends work and other things it takes its toll. So maybe trying to enroll in a team or individual sport, this way you can meet new people and have fun whilst you keeping fit. Running around with people you know is sometimes far better than doing it by your self, this way once you are involved with other people you get an urge to "not let the team down".
For those of you that are independent and like being by your self try a gym membership, that way you are in a controlled environment and you can meet new people, but at the same time if you just feel like being alone put on some head phones and go for a 10 minute run on the tread mill.
There are plenty of options so don't be lazy.