Some one with experience in law enforcement i have a career question?

well i want to be a cop im 21 always wanted to be a cop never got in trouble anything like that the thing is i really don't know how because what i have looked in my area which is missouri is that academies cost $4000 dollars and then the training is every weekend for a full year and that's to long and also i don't want to be just a cop i want to work my way up to detective either homicide or narcotics. so with what ive said what path do i have to take to make a career out of this?