in order to gain mass, you must make your body find use for that mass you're passing through your body.
hence, you should work out.
exercising your muscles is a great way for your body to grow and stuff.
i used to be like 115ish like most of you, 5' 9" and that. I started working out daily and put on 15-20 pounds. do like 100 pushups, 200 situps, curls and stuff everyday and then finish each off with chocolate milk and youll get there in no time