My guess is that you haven’t hit puberty yet. It’s during that period that you will experience at least one, maybe more than one, growth spurt, and until this occurs you should wait with adding another pitch to your arsenal and concentrate instead on building up velocity on your two fastballs, especially the four-seamer. And you need to put on some extra pounds to go with the projected increase in height.
If you absolutely have to have another pitch, consider the palmball. This was the first changeup I acquired, at age 12, and a very good one it was—this is the one they often refer to as the “Bugs Bunny” changeup. It’s easy to throw and control, and you can change speeds on it. Basically, you grip the ball with all four fingers on top and the thumnb underneath, cutting the ball in half so to speak, for support. Hold the ball well back in the palm of your hand, hence the name—but don’t grip it too tightly, because you don’t want to squeeze the juice out of it! And you can vary the speed by tightening or loosening the grip, and/or holding it a bit more loosely. And you throw the thing like a fastball, no ifs, ands, buts or bases on balls. That’s all there is to it. A reminder: you need to throw all your pitches with the same arm motion and the same arm speed. 8)