Don't judge the "yoga people"
I know yoga people seem weird, and maybe they are a little. They chant. They ohm. They wear stretchy pants. They speak Sanskrit during class, preach about the endless love of the universe, and some of them talk about the healing benefits of sweat lodges. One thing they’re not doing is judging you for coming out to practice with them for the first time. Return the favor, because of karma or whatever.
Don't eat two hours before
If it's your first class, you're probably not going to be standing on your head, but you'll be a lot more comfortable in general if you do yoga on an empty stomach. When lying on your back with your legs stretched out, the instructor may ask you to bring one leg (or both) into your chest. They don't call this the "wind-relieving pose" for nothing.
There's no such thing as indecency, so get over it
There's farting, crying, vomiting, wedgies, nip-slips, pools of sweat forming around people's mats, and some overachievers who go for the headstand during their first class and loudly flop right on their backs. Just keep practicing. I've seen it ALL, and it's happened to the best of us. Don't feel stupid if it happens to you, because no one else in the class will bat an eye at it -- unless you fall, then the teacher will quietly come check to make sure you're not concussed or something.