How to gain muscle (a teenage girl)?

if there is a cross fit course for teenagers in your area or something like that, near mine we have BASICS and it helps with strength training. also doing things like wall sits and planks and crunches help. so does running ladders and i dont know if you've heard of a stadium of stairs but running one of those helps with leg muscles. and push ups and tricep dips are always a good thing for arms. hope this helps:) i am also really small and building up muscles, in two weeks ive gained 3lbs in muscles! just by doing some of these things really frequently and playing lots of tennis and being really active. so hopefully this helps!:)