Some of the guys here don't mind carrying around quite a bit of fat, so I'd take their "what fat?" comments with a grain of salt.
That said, a really young guy shouldn't be going on many diets. You should address any perceived shortcomings in your bodyfat ratio by working out smarter and/or better, and by changing what you eat more than cutting it back. If you eat right, you can even improve your bodyfat ratio while eating substantially more. That's part of what some of the "What fat?" guys are getting at. Some of them. Some may simply want mass at all costs, maybe even including steroid usage, so it's right for you to modify your own goals according to what you learn and experience rather than just adopting the goals or fanatacism of somebody else.
By doing less endurance exercise, you may experience a temporary setback in working toward being lean, but you will probably also make it much easier to gain muscle. That muscle itself burns calories and will help lean you out. Paradoxically, you have to eat more to gain that muscle, though. And if at all possible, it shouldn't be crap or part of a fanatical diet plan.
You may also gain a little fat. If you're gaining muscle, though, that doesn't mean your lean mass to bodyfat ratio is necessarily plunging, though. When I was a teenager and very skinny, I found it took a certain amount of muscle just to more easily get more muscle. It's hard to get in a good work-out doing push-ups, for instance, if you can only do 15, know what I mean?
You don't have the musculature yet to need to lean down, because it's more in the stage of "lean down from what?" You should establish some general fitness goals before getting into specific ones. For instance, "I really want a split in my biceps" would be inappropriate for a kid who finds it hard to run a mile or two or rattle off some pull-ups and push-ups and knee bends right after each other, and touch his toes, without much straining or being too winded.
So get some general physical preparedness going. The aerobics are fine, but if your heart is set on getting some mass on you, it can be counter-productive if done in the same muscles you want to build up. You can get admirably aerobically fit with a jumprope in 15 minutes a day, and that will take the strain off virtually everything but your calves, making it easier for you to build muscle over your whole body. And jumping rope is plenty tough calf work for many, so you don't lose out on that score either.
Stretch -- more than you usually see people doing. Keep in mind that most people you see will get injuries, and some will get many. Lack of flexibility is either the direct cause or the indirect cause of an awful lot of that. You want to keep the suppleness of a young body and improve it while it's easy, not get it too stiff. Flexibility is much, much harder to get than to keep.
Concenrate on exercises that emphasize the full body, or as much of it as possible. Not many things do, besides swimming, but if we're talking weights, there are favorites that work large numbers of muscles in concert, like deadlifts and squats for the lower body, and side presses, benchpresses, and others for the upper body. Keep the body working together so you learn to coordinate power throughout your body instead of getting a disconnected chain of uncoordinated powerful parts. If you want to work your triceps, in other words, do some parallel bar dips if possible before concentrating on tricep push-downs. Don't waste time and energy on leg extensions when you could be doing squats, a much more full-body movement.
After you build up a certain base level of athleticism in your body, it will be much easier to get rid of any fat you have simply by keeping on doing what you're already doing. If you go to work on aerobic endurance activities and dieting to get rid of your fat now, you will make it very hard to put on muscle, and that will make it harder for you to get enough muscle up to the levels of intensity in work-outs that really burn fat. Even a brutal aerobic work-out needs some muscle mass to actually be worked out, or the body just burns out early and shuts down before enough work is done.
And even at a peak of aerobic fitness you will still be relatively small and weak, if you got there through doing aerobics and light work. And believe it or not, the worst slap in the face is that you may still have trouble getting rid of that last bit of bodyfat that's bugging you, because you don't have much in the way of muscles constantly consuming calories while your body is at rest.
So build up the body first. Gain some strength and flexibility and aerobic capacity to get a more generally fit body, so you can take on the workload of an athlete. Don't expect to gain much size in the muscles you work aerobically; that's where my suggestion on the jumprope came in. Ten minutes of jumping rope is the aerobic equal to half an hour of jogging. And, well, a hell of a lot faster, eh? Try it.
Gradually work your way into weights more and more, keeping to the ones that use many muscles at once. They will be good for you for a long time. Add in isolation exercises only to the extent that they don't take away the energy you need to do your other exercises.
Eat plenty, sleep plenty, cool out if you can on the booze or drugs or playing with yourself or whatever your bad habits are. Your workouts will be substantially a waste of time if you don't eat enough and get enough rest. Don't compare yourself to anyone else or expect instant results. Don't jump in and out of programs every two weeks; find one or two you like and stick with them, maybe alternating them to keep your body guessing and growing. Don't fall for every new super-theory or mega-supplement of the week that comes down the block.
Read lots. Make excuses infrequently. Let things take the time they take.
You'll gain some fat, and you'll lose it too. As long as you're maintaining good habits, don't sweat it. Remember, building an athlete, or even just a good looking body, is a very long-term project. Pretty short compared to living a life, but if you've got no patience, either grow up or come back when you're ready to make a commitment to what's basically a healthy lifestyle, not a fad or temporary ego boost.