Whatever you've heard about it, I would encourage you to do your own research. Masturbation in itself isn't harmful at all, but frightening children about it and making them feel horrible probably is.
There is no mention of masturbation in the bible. The claim that it's a sin is based on the misinterpretation of a single passage in Genesis chapter 38. Try finding someone who can explain it to you and understands the context in which it was written. To make a long story confusingly short, the sin described had with a man failing to conceive an son for his recently deceased brother, as was the custom at the time.
One its not as bad as all these people are saying and although the bible says its a sin the bible also says many things that seem to not fit today as long as it isnt messing with your daily life then there not really a problem. You are kinda young but the truly only way to stop is though will. You just have to not let yourself do it. But its natural and its part of your bodies urges so its really going to come back.........like i said it more socialy harmful and thats only when done to an obsessive amount
try sleeping with a wet towel between your legs at night for 30 days to help you break the habit.