This is a fantastic book. Buy it if you haven't already. Fantastic book. Really, one of the best books written in any field. Written by Arnold Schwarzenegger. Highly recommended.
Dont underestimate the diet. You can waste a lot of Gym Time & Effort by having a shit diet. That time and effort lifting things will be 1/2 as valueable if you dont eat properly.
You dont have to go Dwayne Johnson on it, but eat a little cleaner, get a lot more protein, dont be worried to put on a little pudge (if you're a skinny kid). You can quickly lose the fat, but the muscles are impossible to gain without the food.
http://www.amazon.com/The-New-Encyclopedia-Modern-Bodybuildi...
I'm excited for you. You're embarking on a great journey, you don't even know! Or maybe you do. :)