What supplements from GNC can help me gain hard muscle and be really buff but at an affortable price?

Ok, i'm 14, but i'm going to turn 15 in June. I have ALWAYS admired bodybuilders since i was a little boy, just the way there did it as a career and how huge and strong they were just surprised me and made me see them as superheros. I wen't to Venice beach with my friends for spring break on March, and i wasn't in any sports or athletic programs, i was just in our school band and choir club. Anyways, we were all going to go to the beach and just hang out there, and it was me, 2 of my best girl friend and 3 of my best guy friends. I didn't feel like wanting to take my shirt off because i was a little shy of my body and since i;m skinney, i didn't want to be seen as a pile of toothpicks. Instead, while there were all changing, i just had a regular T-shirt, my swim trunks, and my aqua shoes. The girls were the first to come out, and they looked nice, but just a couple of secondes later, the guys came out, and i was soo surprised from what i saw, REALLY! I thought that they just had average body figure, but i was wrong, they looked so ripped from there shoulders and even to their feet. The girls were also surprised but they were also turned on and the ENTIRE day they were hanging out with them, and asking them to flex, pose, and even asking them if they can feel their abs. Since springbreack i've been trying to go to the gym and trying to workout, so far i have seen some results, my biceps are now 16inch, 16inch shoulders, 13inch chest, and 14inch calves. I am very proud that my progress has paid off, but my friends are suggestion that i get a boost at it from using supplements that will help my muscles grow a little bigger during my workotus. I have never been to GNC, but i don't just want to go and looking around like a retard. I really want to keep getting bigger and much stronger, and having a really musclar and buff body will be good for me because i'm thinking about joining for some bodybuilding (teen) competitions. If you can tell me what products/supplements that can help but are also affortable to buy and use, that would be great! Please leave ANY reconmendations of supplements or even tips i can use, because i really want to gain much more progress in my body and be able to sign up for a bodybuilding compeition.