We know that most, if not all, officers attended Starfleet Academy. But what of the lower ranks? Memory Alpha states the Academy does have an enlisted program, so that's one route. Are there others?
There doesn't seem to be a mandatory conscription or draft system in effect.
Could commercial or private schools be offering some sort of vocational degree meeting those requirements? Do you just walk in off the street at the corner Starfleet Recruiting Office?
I have no military experience at all, so I'm trying to figure how they convince civilians to enlist. Let's assume they don't always send out a ship captain to rescue you from a bar fight.