is the best introduction I've found.
The basics are as follows, but everything varies by region:
Cost - somewhere between $15-25, depending on whether or not you're an SCCA member.
Usually held on Sundays, usually last from early (8:00?) to early afternoon, (1:00-500?)
You will need to "work the course". That is, you'll need to put cones that have been knocked out back, or some other similarly easy task your first few times. Everyone who races has to do some kind of work, but it's nothing difficult.
You can get anywhere from 2-8 runs, usually 3-4. Sometimes after all the official runs are done you'll get some fun runs that don't count in the season point standings.
Most regions have roughly a dozen runs each summer, so it can be about once every 2-3 weeks. Again, this varies.