First, welcome to VMSC's website. We have a couple of good references linked here that will tell you all you want to know - and probably more. You can find them here - http://www.vmsc.org/links/autocross-reference-material.html
Rather than just reading, I would suggest you come to one of our autocrosses and either watch one first if you have never seen one, or just jump in and participate. You do not have to be a member to watch or drive. Non-members pay $30 to drive while members pay $25. Watching is free!
Our next autocross is June 15th at Richmond International Raceway (RIR). You can register on-line by clicking the registration link at the top. (It will ask you for the class of your car. If your WRX is stock, then it goes in D Stock. If you have made modifications, tell us what they are and we can tell you what class you are in.)
On the day of the autocross, the gates open about 8:00, you can register from 9-10:00, there is a driver's meeting at 10:30, and we start the autocross at 11:00. Just identify yourself as new to autocrossing at registration and someone will explain how things work. That is how I started about 6 years ago and the only thing I have regretted is that I didn't start sooner.
Fire away if you have more questions.