Hi, I'm 16 years old and have just begun working out. I'm extremely weak compared to others (112lbs!). For some reason, it's hard for me to gain mass. Recently I've begun working out with 20lbs. dumbells. I do roughly 12 lifts with the dumbells on each hand (4x), and then as many push ups as I can (20 or more at least), once a day. What am I doing wrong? Should my workouts be more intense? I'm barely gaining weight, so it's obvious that I'm not gaining muscle either. Any help would be great.