I couldn't get over 140 lbs before I was 25. I've always been an active person and played different sports and I still ride bicycle everyday. I could eat anything and not gain one pound. I don't know exactly why that is, but probably has something to do with metabolism.
Now I'm 27 and I'm in the 200 lbs area. I'm 6'1, so I'm pretty tall and not overweight at all. I started lifting weights in January, when I weighted around 160, so I gained 40 lbs in 10 months.
One important thing I learned is to watch your calorie intake. When working out, you burn calories, if you burn more calories than you eat, you're not gaining muscles at all, you're just burning fat and if you don't watch it you can even burn muscles (not likely, unless you go crazy on your work out). So, the most important thing to remember when building muscles is to eat more than your burn.
I'm not entirely sure what you're aiming for, but I'm guessing you want to look ripped and have abs you can grate cheese on. That's what I'm aiming for. You can look ripped like Style, by just not eating that much and some light workout. He looks kinda frail, so I'm not sure you will want that. I suggest you start counting calories and make sure you don't expend more than you consume. Combine that with some (heavy) weight lifting, say 3 x an hour every week and you'll be building muscle mass pretty quick. The best result for me was when I started lifting weights I couldn't lift more than 10 times in a set.
After a couple of months you'll get buff and then you will want to start cutting. Get definition. Then it's a simple matter of burning more calories than you eat. Best to combine with cardio to stay in shape.
As far as what to eat, I suggest natural food. For protein rich food, it's fish, turkey, chicken and other lean meats. Also yogurt. And whole grain products.
Also, read this http://www.liamrosen.com/fitness.html