First I'm a 16yr old male, Have a 28in waist circumference, 5'6 and weigh 110Ibs. I used a calculator online and it said that I have a 12% body fat percentage. I get comments all the time saying that I'm to skinny and underweight. Even my doctor prescribed me protein shakes to drink 3 times a day to gain weight because of my overactive thyroid/metabolism. Anyways, I want to start working out starting today and would like help understanding how to gain muscle mass and maybe a six pack. I do work out in my gym class's training room(mostly bench press and bicep work). My stomach does have some fat on it, but on the side of my chest you can almost see my ribs. I just want to shed fat off my stomach(which I know I can't target directly) and work on my abs, but still want to be on the safe side of weight because I don't want to go full skeleton lol. ANY HELP WOULD BE AWESOME! Thanks!!