sgtsixpack is right, your not going to shift your weight by doing pec deck, shoulder press etc. Ok your will feel like your working but you need to cover the whole body, as often as you can, as hard as the can for as long as you can.
Also your 15, forget about the 6 pack for now you have loads of time, im 24 and only now can i do with my body what i want, just get the fundamentals right and you will be flying.
I dont know what area you live in? but certain gyms are going to be better than other in terms of those willing to show you stuff.
Body buildng just isnt the way forward for you at this stage, do some but it shouldnt be your primary focus.
Oh and sort your diet out, diet coke???? cut that shit out, get yourself some meal replacements, whey protein, cut shit like potatoes, pasta etc if you want a more rapid result and eat meat, chicken, fish, steamed veg, and low GI fruit i.e. green apples, green pears, banana's.
Eat as often as you can, as much quality as you can and at your age you can get your metabolism flying!
Hope that makes sense