Question

Yes, most workouts are targeted for adults, and while you are not legally an adult... your body is ready to be one. I began training (ie. lifting weights) at 12 years old. I've done martial arts since I was about 8 years old... almost 40 years later I'm still doing the same things. Train properly (technique, frequency, intensity, duration, etc.) and all will be fine. If you have a knowledgable coach in your area he may help you. Or even better... find a martial arts academy. The best martial art is the one you enjoy and will stick with for years to come. I recommend Brazilian Jiu Jitsu, Sambo, Judo, Muay Thai, or MMA. BJJ is definitely number one if that is an option.

At 14 you probably have little to say with what food is being bought at the grocery store, but you can moderate the amounts you eat. For now, begin training in one of the ways stated above, and if fat is not coming off then begin reducing what you are currently eating. Obviously no candy, no sugary sodas or drink (i.e. juice), and in general as little sugar as possible. From there, you can begin reducing grains or other carbohydrates. Keep it simple and you'll be okay.

On a bigger note, I can not stress enough how import and life changing it can be for you to get involved in a challenging and/or competitive athletic endeavor like martial arts. You will likely find that your confidence will grow, and your unhappiness with your appearance will subside. Your body is simply a vehicle that moves the real you about in this world. There are plenty of people who walk around with ripped abs, but disgusting character. Focus on developing character, discipline, and kindness and it will all workout.

All the best.