First things first, you've gotta eat. Whatever you're eating now, increase it by 10%, while trying to optimise how healthy the things you put in your belly are (healthy by means of being nutritious and containing minimal doses of poison, not by means of being low-calorie, as you need calories to grow).
What equipment do you have access to?
http://training.fitness.com/weight-training/basic-novice-program-strength-muscle-54545.html is a good program for learning basic movements and getting bigger and stronger at a beginner level, however you may not have the equipment required for it, so you may need to make modifications or change the plan entirely.
For adults, once the movements are learned to a passable standard, I'd generally give the advice of training until you're 1-2 reps short of failure, and adding weight as often as possible while maintaining/improving technique and sticking to 1-2 reps shy of failure. Since you're a young teenager, I'm not going to give that advice. While people perhaps go overboard on their fears of strength training being harmful to children, they aren't 100% unwarranted, either. Basically, I'm going to be twice as conservative with you as I would with an adult. I want you to focus on developing good movement patterns and technique first and foremost, and to finish every set 100% confident that you could have done at last 2 more reps, and reasonably confident that you could have done 5 more. This will mean slow progression in weights. It will also mean slower development in strength and muscle mass. It will also mean a very low chance of your mother suing me for giving you advice that got you hurt; and I am highly in favour of not getting sued by your mum.
