No contracts until you either get a scholarship, or when you come back from SFT (summer field training) the summer between your sophomore & junior years. So unless you get an AFROTC scholarship, you can leave anytime you like your first two years.
There are some cases where scholarships are given for specific jobs. The most recent was a Missileer scholarship. Make sure you learn as much as you can about these jobs before you sign. Otherwise, you can end up choosing your job either in your junior or senior year.
For uniforms, that differs from det to det. In my det, you have to earn them, so it can take a semester or two before you earn both uniforms (blues & ABUs). Other dets seem to give them out willy nilly. Either way, youll be instructed on that when the time comes (probably sometime in august, so long as they know you are joining).