umm .. you can discuss with a recruiter what you would like to do, if there is a demand, you can have your AFSC when you sign up.
(you have to get passing grades in Tech School to keep it, natch)
Some career fields will pay a bonus for enlisting in them (and re-enlisting if a career is decided on)
You can also enlist for 6 years and exit basic as a two striper with 3 stripes awarded at completion of your first year in service..
..provided you don't punch out your CO or some such

IMHO .. it is *much* better to get a degree (in anything you enjoy) *then* enlist as an officer candidate.
Much better incentives all across the board.
In any case ..ya gotta do the research ... goin in blind is askin to get screwed.
-GE aka Frank
(ya .. I told my recruiter I wanted to learn how to install speakers in my car
.. he said there was a bonus for goin into ECM (328X3) and if I went for 6 I'd be an NCO in a year.
..I did, got passin grades thru tech school, and I was)