I was skinny growing up and through my 20s---was 160 when I graduated highschool and about 170 when I graduated college (and 6'1"). I lifted a bit and never saw many gains. Then, my 30's came and I stated seeing weight gain so I used the opportunity to hit the gym. In two years, through gluttony, age and exercise, I have gained and am not at about 195 pounds.
Drinking water, eating fiber, fruits and veggies and lots of protein helps build muscle mass. Also, you have to let your body rest-muscles need time to recover and grow. I work out 4-5 times a week for 40 minutes. 15 minutes of cardio on a bike (fat burn, cardio, hill) and then a different muscsle group each day: Chest, arms, shoulders, back, core).
I hit a wall about 4 months ago--weight wa stable but muscles weren't growing, so I started eating spinach salad with albacorfe tuna/chicken breasts on it, more yogurt, almonds, etc.--and was ab le to see shifts in my physique as a result. I have never taken any pill or supplement, although I know they work for some people. Just my thoughts.