Various body/fitness questions? Trying to be more healthy & gain weight!?

My answer is not going to be as detailed ad the guy above me but what I'm going to suggest is eat healthy and drink protein. It feeds the muscle and is critical to drink while or within 30 mins of a workout. Notice I said workout cause that is critical to gaining weight as well and it is up to you to gain weight because 5'5 107 is small for a 17 year old let alone a 15 year old. Change your diet and your habits and consider lifting weights either with a friend or join a gym.