I have a question about marijuana and becoming a police officer in California?

I am a 20 year old male who is currently attending college, and is wanting to one day become a police officer. However, when I was 14 years old, I smoked marijuana just one time. I consider it to be the WORST mistake I have ever made. I have never done any other narcotic and I don't even drink. However, I know that when I try to get hired (as a police officer) they will do a psych-eval, and a polygraph. Is that something that I want to be honest about? And will it keep me from getting hired? I have NEVER been arrested or in trouble with the law, but I did make a mistake or two in high school. However, in my adulthood I am a nose-to-the-grindstone, hard working person, and I am a very hard and fair worker and student, and I have plenty of references that would confirm such. Would the mistake I made in high school prevent me from getting hired as a police officer? Or do they expect people to have made some mistakes at some point in there lives? Please, I really would appreciate good, educated answers. Thank you for your time.