How to gain weight/gain muscle?!?

Alright, I am 15 years old, 5.8" tall and only weigh 105 pounds and can barely bench 90 pounds!. I really need to start getting bigger beacuse i am extremely skinny and scrawny. I use to eat a lot of healthy food and I went to a personal trainer over the summer and did not help at all. But now to make it even worse, I have to take hdhd medicine that makes me not hungry at alll and makes me extremely inactive but I have to take them for school. I just would like to know about a pretty simple workout that I could keep doing eveynight to get a little bit bigger. Also, I really do not want to have to work out for too long, like at least 20 minutes because if I workout for too long then I will throw up cuz I don't eat. I know that I have to eat a lot of food to get bigger and I will start eating a lot but I just don't wanna be throwing up because my pills are really strong and will make me throw up. Just can someone help me out with good 20 minutes workout ideas and some good foods to eat pleaseeee!!