Requirements to become police officer?

What education, degree, training, etc. is needed to become a Police Officer? I would like to do it in the St. Louis area, specifically O'Fallon, MO (I don't know if that makes a difference). Would it help if I was a volunteer officer first? I'm 16 years old, I'm guessing I'd have to be probably 18 if I wanted to volunteer (If they even have volunteer officers). Please help fill me in.