How to gain weight in a healthy way?

im 17, 5'6, and 112 pounds. im very active. i do karate, kickboxing, tennis, and track. the only meat i eat is chicken. i wan to gain weight just so i can look more like my friends my age. i look liike i never hit puberty... 32 A... no s. stick legs. how do you gain weight in a healthy way.. like in muscle ... or fat but not being overweight? just to look fuller?