LOL. I totally know where you're coming from!! I was there a year and a half ago. Like Hex Rei said, be humble, and relax! Don't be the guy who thinks every training or sparring session is a fight. Jump in with both feet and start learning.
Expect a few bruises, sore muscles and what not. They go with the sport. But mostly, expect to meet some of the nicest, coolest people on the planet. Don't be afraid to be out of your element, we've all been there. You're not going to get thrown to the wolves and torn up.
Don't worry about what shape you're in, or what you do or don't know. Just show up and do your best. Seek out people who are better than you and pick their brains. (This is where being humble really pays off!)
Hope this helps relieve some of your anxiety.