This used to be a huge problem for me when I was around 15. I think what motivated me was seeing that all the guys with six packs were nabbing all the girls, coupled with the fact that I'd be out of breath every time I reach the top of a flight of stairs.
In the beginning, join a gym (or if you have a treadmill at home, use that. Or if you prefer to jog outdoors, do so) and visit it 3 times a week, jogging for half an hour. I noticed that people who start out easy and gradually move on stay with it longer than people who start out full blast. Those guys just end up burning themselves out. (You could replace the jogging with swimming too. I found swimming to be much more effective). Also, remember to go in rotation with exercises. E.g. a month of jogging, then a month of swimming, then a month of the elliptical, then a month of cycling, then start over. This challenges the body and makes it keep losing weight. For the three exercises that take place in the gym, it might be a good idea to take an iPod a long.
It might also help to start some kind of sport, or some form of physical activity like rock-climbing or something.
The trick is to stick to it. I've stuck to a workout routine that has been steadily getting more and more difficult for 3 years now, and I'm in a million times better shape than before. Now at least most girls would
consider me before rejecting me, which is a vast improvement!

Edit: Oh yeah, I forgot to mention diet. :facepalm:
At first, don't get all hopped up on protein shakes and what not. Just avoid all empty calories or fried foods. So, nothing like chips, or fries, or grilled cheese sandwich (unless the bread is brown). And absolutely NO fizzy drinks whatsoever (No alcohol either). And don't smoke. Treat yourself with something on occasion, but not very frequently. Like, once in two weeks or something.
Anyway, if I missed something out or anything, feel free to ask, as this is something I've done before.