Scroll up. I'm not joking; I was 140lbs when I graduated high school, and only 155lbs (and 6' 1" tall) at 30 years old.
You have to eat a lot and often. Make it a routine; every morning, especially weekends.
My breakfast is some kind of meat (usually lean ham, occasionally bacon), two eggs, and sometimes oatmeal (one serving, which really isn't much).
Lunch should be a big deal. I usually cook several chicken breasts once or twice a week and brings them as left overs for lunch with some kind of whole grain and a vegetable. That's my lunch nearly every day. Sometimes I'll switch to pork or turkey breast, but it's always that same kind of combo; meat, grain, vegetable. Dinner is similar.
I snack during the day as well. Healthy stuff; nuts, fruit, tuna.
Eat, eat, eat.