The fitness advice in this thread seems good. I would add that it helps to do something fun - it's easier to stick with enjoyable swimming or biking or a team sport than a grueling and boring weight routine.
Not to pry about medical information from a kid online, but how overweight are we talking? There's a difference between a few pounds chubby and dangerously obese.
I get the feeling that people on this board see "freediving" and picture the super deep hardcore competitive stuff. That's fun too. But If you just love being in the water and exploring for cool fish or lost jewelry you can do plenty casually snorkeling and maybe diving down a few feet relatively safely. Compared to diving it's a lot less gear, less cost, you can spend longer in the water, and it's a better workout. It helps build water skills, experience, and confidence that will help when you get into SCUBA. I did a TON of snorkeling at your age, and still love it to this day even with my fancy technical diving certs.
Have a buddy with you, read up on basic freediving safety practices, progress slowly. Start with safe and easy spots. Don't overweight your weight belt. Bring a marker flag and a knife or line cutter for possible entanglements. There are techniques to train longer breath hold and deeper dives, but don't get into any of that until you feel good about your fitness and comfort level doing the basics.
I'm not sure how good the snorkeling would be in your location. If you're near any lakes there might actually be a lot of cool stuff to explore in the shallows. Even crappy muddy lakes can have surprisingly big fish lurking near the shore, and lots of funny stuff drunk fishermen drop overboard.