Lifting weights won't do you any harm or stunt your growth as long as you perform the movements correctly. Lifting weights incorrectly can lead to posture issues and a great deal of pain later on in life (and by this I mean in your 20's).
As the others have suggested, it's better to take part in an activity where you can gain all round physical excellence i.e. balance, bone density, cardiovascular fitness etc. (plus it's usually more fun and social).
If your heart is set on hitting the gym (I'm surprised they let you use the weights) then remember these guidelines:
Muscle toning - lower weights, many repetitions, high intensity with short rest periods
Muscle bulk - heavy weights, low reps with adequate rest but not too much.
Follow a healthy diet and keep hydrated too.
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