I agree with mrtornadohead on the shoes/clothing. A friend of mine did his first race in a T-shirt, plaid shorts and sneakers. One exception I'd make is if you usually use clipless pedals and road shoes. I'd go barefooted before I tried CX in road shoes. (I've seen that done too. One of the juniors lost a shoe in the mud, took off the other one and finished the race barefoot.) You'll probably feel like you want to stay warm and dry, but that's not really possible. Wool socks are a very good idea, but sticking with shorts is the way to go. Bike shorts are preferable, as it reduces the chances of hooking your crotch on the seat during a remount.

I also agree with mrtornadohead about Cross Crusade being an amazing series. There's nothing like the first race at Alpenrose. But, if the idea of racing in a field of 100+ riders intimidates you, there are a lot of smaller races. Check out http://www.obra.org for a full schedule. Short track mountain bike races are also a lot of fun and can get you racing in the summer. All this assumes you meant Portland, OR. If you're in Portland, ME you should consider moving to Oregon.

The rules in a nutshell: There's a short course (about 2 miles) which covers grass, mud, pavement, sand, etc. (whatever is available). You have 40 minutes (as a beginner) to go around as many times as you can. Everyone finishes on the same lap. The first finisher with the most lap wins. There will be barriers to force you to dismount and run with your bike. There are usually short steep hills where it is advantageus to run with your bike. For OBRA CX races, any bike with nearly any equipment is allowed. I have never seen anyone pulled from an OBRA race for being lapped. The promoters basically bend over backward to make it as fun and accessible as possible to as many people as possible.

Watch the OBRA calendar. There are already some great CX clinics scheduled, starting in August, where you can learn the basics from top notch racers. I wouldn't be surprised to see more clinics planned as next season approaches.