There is people who did not grow as much because of what they take at the gym which Arnold took. (can you name it? because i have a headake and i cant think of the name)
That chemical has been known to stop the growth of the bones.
Go n do some weights but take it easy, start light, and if you can do 6 with no struggle, next one up and repeat. Once you feel the weight, try to do 10. this is a rough guide but its harder to find your 1 rep max alone, find a Personal trainer (PT) and mention this is your first time and what is the ideal weight for you to start, if the person has time and personal opinion here, cleaver to do you a one rep max to find your heaviest weight, that will be great so you know what you can not lift without strain.
REMEMBER: Muscle rips, they repair slowly for beginers for weeks maybe so it can become stronger. If a ligament rips you done something wrong and that takes longer to heal. Dont worry though. TAKE IT EASY.
Youtube is good but some people who are body builders will show you their method but thats to get what he has and im sure your not up for looking like some MR MUSCLE OF THE WEEK at 16. Your PE teacher should be able to advice you too!