If you've never autocrossed before, expect to get beat. Don't get upset when it happens, and don't get upset by the kind of car that beats you. My first event, I was dead last for the entire event. I was in a Fox-body Mustang that could run low 12s at the dragstrip, and I got beat by a Plymouth Sundance -- yes, a K car. True story.
The kind of people? It depends on how you present yourself. If you are willing to learn, asking questions about how to drive, and what pressures to run, etc., they will be really friendly. If you bring 20 friends that all want to ride with you, drift around the course, drive like an asshat on the roads leading to/from the event and generally act like a stereotypical ricer, they likely will ignore you at best, and at worst, the organizers might ask you to leave because you and your friends are a safety/site-retention hazard. My first event, I was too shy to talk to anyone. But no one really went out of their way to talk to me either, which is a shame. I've found that a lot of novices want help, but are too shy to ask for it. Most autocrossers are willing to help out -- either by letting you ride along with them (if the club/site allows ride alongs), or by giving you at least some set up advice on tire pressures, alignment, swaybar settings, etc. Some people won't give all of their setups away, but I'm not one of them, since I feel that autocross is more about the driver than the setup.
If you're in the NoVa/DC area, you should check out Autocrossers, Inc. events in Waldorf. They are very novice friendly, and have many SCCA National Champions in their ranks (myself included). Washington DC SCCA has events at FedEx Field. If you are elsewhere in Virginia, the Richmond/Va Beach area is served by Old Dominion region (ACU 4 is an awesome site on the beach; it's a hovercraft base), and SW Virginia is served by Blue Ridge Region SCCA (my region of record and where I started). There are some excellent regions in New York also.
I wouldn't worry too much about modding the car until you learn the ropes of driving. Probably the only things you should be concerned with car-wise are alignment and tire pressures at this point.
Good luck!
Karen