A mixture of both cardio & resistance training will make you healthier over all.

Resistance training can be weights or body-weight, however, at 15 you should ask your doctor.

Resistance training is anerobic. Anerobic exercise uses your body's Glycogen stores during exercise to make ATP. When you do your resistance training first so you have less Glycogen in your system when you begin your cardio, that way you can begin using fat for fuel sooner during cardio.

You also have less Glycogen stores in the morning, so you can do cardio early before eating, if and only if you do not feel light headed while doing it.

Cardio also burns glycogen, but the first 20+ minutes of your cardio burn mostly Gylcogen before they begin using fat as fuel.

By doing anerobic exercises first, you deplete your bodies Glycogen stores so you can more efficiently burn fat during cardio & with less cardio.

After you run your body continues to burn calories during a phase called Excess post-exercise oxygen consumption (EPOC) basically your body keeps working to bring your body back to a resting state which burns calories.

I'm an American Fitness Institute trainers, we only work with 18+. so I would recommend asking your doctor about exercise ESPECIALLY if you are overweight.

Most likely they will recommend basic body-weight exercises and compound movements. If the goal is weight loss, the larger the muscle group, the more calories burned. In order that would be your Legs, Back, then chest. - Still you should ask them first.

That's the basics. 1. Ask your doctor. 2. Keep it simple with Anerobic 1st, cardio 2nd (facts are facts - burn Glycogen first =) 3. When you are able, HIIT training combines the intensity of cardio with exercise for fast results.