How can I stop thinking about drugs, sex, etc?

I just started going to Public school and I am now in the 7th grade. I used to go to a Catholic school where they had very strict rules and always made it very clear to say no to drugs and things like that. Ever since I came to Public school many kids do drugs and have sex. I think about the "what ifs". What if this does happen to me? I think about what would happen and about my future. Lately, this has become a big issue and I want to know how to stop thinking about and how to refuse it if someone ever asks me to. Can someone help?