becoming a cop

i want to be a cop, but im nervous about 1 thing. Ive been a good kid my whole 17 years of life except for 1 time. Im scared because they are gonna ask if ive smoked weed and idk what to say. I havent smoked weed myself, but i got contact high by being around it. What should i say to that when i apply? please help, im making myself sick im so nervous.