I'm not a doctor, but I've been in the sport of fitness & bodybuilding since I was your age ( and I'm an old guy now). If the problem you have is "left over" from being a little heavy as a kid, and now you're in shape, it's probably a body fat issue and not gynecomastia. It might get a lot better with time and patience. But if you were never heavy, and you've tried hard to get rid of it, and if you've hit puberty and have grown in height & size, and it's still there, you probably need to see a doctor about it. Regular doctors don't always care about things like this, and if your parents belong to an HMO insurance plan, you might have a problem getting permission from the insurance company for this kind of problem. But with that said, a doctor will be able to tell exactly what's in there, and will recommend how to get rid of it. If surgery is what's recommended, don't worry, it's no more serious than having a tooth out. Some procedures are just a needle to suck out fat, or if you DO have adult-type gyno, they numb you with novocaine and just make a small cut under each nipple, and pull it out in 1 piece. You don't even have to stay overnight, and if you're OK with being awake when they do it, it's not a big deal at all. You can see gyno surgeries on YouTube. But I have the feeling that most doctors won't want to do that for you until you're full-grown, in case it might go away by itself. But I sure wouldn't want to wait another day if I were you, so I would want to push hard for help! Sometimes dealing with doctors can be hard & disappointing, and that's because most doctors can be narrow-minded about things like this, and think you should like your body the way it is, and only have surgery if you have a medical need for it. Bullsh*t! A problem like yours can totally wreck a young man's ego, and drive you do depression, or cause you to feel bad about yourself and get you a rotten girlfriend/wife later, and I could go on & on. You're smart to have gone out looking for answers, and I think you'll solve this. I know I'm off on a tangent with that last comment, but believe me from personal experience, things that bother you at age 15 will breed more, and bigger things if not worked with early. Self-image is very important, and it's fragile enough as a teenager without some kind of a medical problem that embarrasses you and makes it even harder to strengthen.
As far as trying to figure out exactly what your situation is before you see a doctor, you can do this: My understanding is that if you're in good shape (low body fat) and you have gyno, you can probably feel a jelly-like glob in there. At the edges of the glob, you just feel skin & muscle. If you can't find an edge, you might have a fat deposit even if you are in shape. Liposuction can fix that, and is probably the best bet in that case.
Let us know how it goes...
Charles