As idkfa said, body weight excercises are your friends. Lots of push-ups, sit-ups, pull-ups, etc... Do random variations on all the exercises that you can use your own body weight for and you'll get stronger, hence have more muscle.
Like others have said, don't go by weight, go by inches. I'm about 6'2" and I weigh somewhere in the 230's. I've got a bit of a belly (about a 37-38 waist) but I'm just a big dude. Stay away from heavy lifting until you're 18 or so and just work on cardio as well as doing free exercises.