When I was 18, I weighed 115 pounds at 6' tall (no, that isn't a typo). When I joined the Army, they made me drink a gallon of water to make the minimum weight for my height. I'm now about 230, and the first 85 pounds came without gear. Mostly by eating and squatting.
I have 3 specific suggestions that I wish someone had given me 15 years ago:
1. Eat more (ok, so that's a very general suggestion). Make sure that the FIRST thing you do in the morning is to take in protein and carbohydrates. The LAST thing you do at night before shutting your eyes should be to eat some protein. Make sure you are never more than 4 hours without eating. Take some time to really figure out what it is that you're eating (macronutrient breakdown). This can be difficult if you are eating a lot of Asian foods (find me a calorie counter that has kimchi in it!). Work on gradually increasing the calories, over a period of weeks. Your stomache, and your metabolism will adjust.
2. Track your workouts and use progressive resistance. Take a notebook to the gym. Write down every set you do. Each week, work on making SMALL progress. Make use of those tiny little discs. Stay in the 8-12 rep range, and work on adding either 2.5 or 5 pounds to the bar each week. If you don't complete the reps, stay at that level until you can. One of the problems I had early on was not understanding the actual intensity of my workouts. In one week, I'd do, say 135 pounds for 10 reps. The next, I'd slap on 10 more pounds, and only get 7 reps. Did I actually lift more? Not really. 10x135 is roughly equivalent to 1x180, while 7x145 is roughly equivalent to 1x176. Try NOT to mix your exercises up too much. You want to get better at the specific exercises that you do, so that you can handle more weight with them. More weight, more mass. When you start to put on some size or get seriously stuck, then you can think about changing things around.
3. Squat. They hurt. They suck. But they work.
