If you want to get big, get yourself on a high-intensity workout program. Start slow and work your way up to regular training. Try to set aside an hour 4-5 days a week for training.

Do some cardio, I like stationary bikes myself. 20-30 minutes during each workout. Then, its on to weightlifting. Take time to learn how to weightlift properly, make yourself a sensible workout schedule... then add weight until you reach a low-rep, high-intensity schedule. Add weight as you improve. There's info all over the internet on how to do this, do your research so you don't end up hurting yourself.

With all this exercise, your body will crave nutrients to repair and strengthen itself. Get on a good diet... lots of protein, vegetables and whole grains. Work out and get lots of sleep. Once you get in the rhythm, you'll feel the difference in a few weeks, and start noticing the difference in the weeks to come.

Some people just have rabbit-like metabolisms, and they just can't seem to put on weight. Trust me though, if you start working out seriously, your body WILL demand more food and generate mass to offset the demand on your muscles.

I know a lot of people who supplement their diets with extra protein (via that weird gray chocolate powder junk), which is not something I do. Granted, when I work out now I don't accumulate size as quickly as when I first started out, but I'm fine with it. I also eat a stupid amount of chicken and fish, which is delicious. Nothing like tuna steak and eggs first thing in the morning.