At 14, you have a lot of potential for amazing gains. The first step for young athletes in jump training is to develop strength. This is done in the weight room with squats and other lower body lifts. If you don't have access to weights, 1 leg sqauts, 1 leg stiff-leg deadlifts, and pushing a car are good things you can do without weights. There are other types of training, but they won't be effective until you have strength. See
Vertical Leap Training Also, it sounds like you may not be terribly athletic, so make sure to keep playing basketball and other sports to develop coordination and all around athleticism. Good luck.