Yeah, like ian said, diet is extremely important in lifting. You can lift day and night, but if you aren't eating enough and not eating the right things, your progress will be poor.
Anyway, it's good that you want to get into this early. I wish I was in your place. At your age, you could see some exceptional progress over the first few years, compared to the progress guys over 21 might be able to expect.
Just take it slow at first, make sure you maintain good form and learn the core lifts. Don't go too heavy at first, not until you have the coordination needed to handle heavy weights.
I say this to almost everyone, but in your case, I would really recommend that you stick to the main compounds. You want to build a solid foundation of strength and muscle for later in life. I would focus on squats, deadlift, bench, bentover rows and military press. That will get your whole body strong. It's also good for your bones, since you are still growing. As we all know, lifting weights is a good way to prevent bone problems later in life.
Being so young, I'm sure you could add a large amount of muscle in just a few months, maybe lose your gut at the same time. What is your current height and weight, by the way? You'll need that to figure out how many calories you should be eating per day.