I'm 14, around 189cm tall, 72kg, slender build and I have a record of around 7 for the beep test. I want to be physically stronger to play better rugby. Like strong shoulders and big muscles. At the moment one of my hardest shoulder's into a guy around my build just makes him stop dead in his tracks and almost fall over. I want to make him to smashed to the floor. I've tried a little weight training with my 20kg dumbell set. Any tips?