Matt said a lot of good things. But I think the info on tire pressure could use a little clarification. On non-race tires, lowering pressure will decrese grip, since the car will tend to roll onto the sidewalls (that's where all that screetching will come from), adding air pressure will increase grip, since there will be less sidewall rollover. Start out at say 42 psi all around. After each run or two check the pressures, you will likely have to bleed air out of the tires since the pressure will go up as the tires get hot. With non-race tires, usually you will want a little more pressure up front than in back say 44f/42r to help the car rotate.
http://www.subrew.com/ has a great autocross primer with a Subaru tilt. I would disconnect the MBC and run in a stock class the 1st few times out. Also, remember to ask questions... most people at autocrosses are very cool with helping new drivers get into the sport.
Reno SCCA has a nice intro FAQ about SoloII here:
http://www.renoscca.com/intro.html
that includes stuff to bring, and basic setup info.
Good luck!!