There is no way of knowing if this is a joke, or not, but there are defiantly kids out there asking this question.
I say:
I know that having a six pack can be a great thing. Makes you look cool in front of your friends, and all the girls whisper about you at the birthday/pool parties, but…what happened to just being a kid.
You are very thin. No one will dispute that. You do need more muscle mass. No one will dispute that. But you don’t need it right now. You’re not even 15 yet. Your bones haven’t even stopped growing.
I’m glad that you’re taking such an interest in exercise though. More kids should, but you need to be focusing on schoolwork.
In my opinion, you are probably neither physically, nor mentally ready to put in the commitment to your training, and diet needed to make the progress that you’re asking about.
As for any advice that you’ve gotten about lifting heavy…FORGET IT!!
You aren’t even 15 yet. If you want to workout, then learn the proper ways to do sit-ups, push-ups, chin-ups, and movements that utilize functional strength.
You’ll build a great base, impress your friends with the amount of pull-ups you can do, and since your body fat is no doubt very low already, you’ll see your abs in no time.
These are my opinions as a personal trainer, and someone who has a brother your age, who I would NEVER suggest lifting heavy weights to.
P.S. Cut out the pop. It’s bad for the gut, and the teeth.