lots of questions here and probably lots of different answers too.
gearing is somewhat dependent on the terrian in your area, and the type of rider you are (your fitness, strength, spin -vs- mash). i have seen ppl run 34:17 and 42:18...its really a matter of preference.
the classes are generally A, B, C, masters, juniors, and sometimes there will be seperate ss class(es). this depends on your area. out in seattle and portland there are alot of ss-ers so i think its more common out there than it is in the northeast. but it doesnt really matter because you can still just jump in the c's or the b's, depending on your abilities.
cx races are timed, so depending on your class it could range from 30 min + 1 lap, to 60 min + 1 lap. timewise, one lap can range from approx 5 to 7 minutes.
for most races you will need a license, but you can get a day license for about $5 if you dont want to buy a yearly license.
best thing to do is just go and start racing. be prepared to get schooled, but dont worry about it. you will have a blast. cx is a great sport, especially as ss!