For ANG and the Reserves, you do know your aircraft and base before you even go to AMS or OTS. That's because you have to apply to the base directly.
I took my AFOQT and BAT senior year of college because that's when I got seriously interested... studied a little with Arco's AFOQT book (you should be able to find it in your neighborhood BN). That book should help you out, tell you exactly what types of stuff will be on the AFOQT. You can't study for the BAT (which is now TBAS) because it's simply a hand-eye coordination test.
Your pilot composite score from the AFOQT, TBAS and the number of flying hours you have add up to a PCSM score... no one knows what percentage of each factor goes into the PCSM score, but obviously the higher PCSM you have, the better, so try to get at or above 90 percentile on all of the AFOQT sections... and btw, it doesn't matter if you have 300 hrs or 3000 hrs, the number of flying hrs they factor into the PCSM score is capped at or around 300 I think so anything after that doesn't matter.
A good PCSM score isn't the only thing they consider though... you actually go to the interview like any other job, and sit before several people... who knows what they want.. a personality that would fit in with their guys, previous military flying experience, etc. etc.