HEAVY WEIGHTS, HIGH INTENSITY
There are many different weight training routines to follow, but to get big you need to lift heavy weights with intensity.
One routine I swear by and have posted here many times is as follows:
6 days/week
Split 1: Back, Chest & Arms
Split 2: Legs, Shoulders & Abs
Set 1 - 12 reps, very easy, almost too easy.
Set 2 - 10 reps, increase weight, harder but easity doable
Set 3 - 8 reps, increase weight, hard should not be able to do more than 9 reps with this weight.
Set 4 - 6 reps, increase weight, very hard, the 6th rep should be the very last rep you could do. Even if someone held a gun to your head there would be no way you could do a 7th rep. (Be honest with yourself, and don't be a whimp. This set should be F'ing hard)
Set 5 - 12 reps, go back to the weight for Set 1, virtually no rest. The weight will look easy. But your muscles are wasted and this set should be very hard.
Set 6 - To failure, same weight as Sets 1 & 5. Again, virtually no rest. Just keep pushing until you can't get another rep. DON'T BE A WHIMP. This is supposed to be hard.)
Sometimes between sets 4 & 5 I will really flex the muscle. No you're not being a show off, you're pushing blood into the muscle.
This whole routine is meant to be extremely hard and intense. There are only 6 sets per body part, that's all you need.
Also make sure you are getting enough high quality protein in your diet, and add some high intensity cardio to keep yourself lean.