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.
I've always assumed it works in a way similar to today's military organizations.
You've got the officers coming from the Academy. There may also be direct-cmmission programs for some medical or scientific specialties as well.
Then you've got folks who enlist in Starfleet, go through some sort of basic training (or boot camp if you will), followed by job-specific trainig for wehatever they'll be doing in the fleet.
Never give up, never surrender! - Cdr. Peter Quincy Taggert, NSEA Protector, NTE-3120