At thirteen, you are probably old enough to start doing some strength training. Talk to the phys ed teacher at your school, or your rugby coach. They can probably get you set up with a weight/strength training program for a beginner using whatever equipment you have available. Failing that, you can gain plenty of strength with body weight exercises only -- pushups, pull ups, knee bends and things like that. You can also gain strength in you legs by putting some load of some sort on a toboggan or sled or something similar, and pushing it about on a grassy surface.
I'm 13 too and like you play Rugby. I'm already 5'10 and bench 65 KG (144LBS). If it's Rugby that you want the muscle for then you should focus on building muscle on your legs. To do this I would recommend doing basic things, like deadlift and weighted squats. start off with a very light weight (20-30KG) and gradually you will improve. After the first month or two, add on another 5-10 KG. As you start using heavier weights, lower the weight you add on. If you're building muscle then you need to do short reps with a lot of sets (For deadlift I'd recommend starting off with doing 5-6 Sets of 7-8 Reps.) I hope this helps.
Gym Or steroids
