Hi man, I really wanted to try yoga and felt pretty much the same as yourself. I turned up and just sat on the floor until the instructor turned up and within a couple of minutes of chatting and getting me a yoga mat she let me know a little bit about the class, showed me the child pose and said if I had any problems with an exercise just go straight into that pose etc.. and it was fine.I loved it and been going ever since. I also found out I could do a lot more than I first thought and I'm sure you will too.
The important thing is to not worry about others, just go in, smile, introduce yourself and go with it.
Dont go carb heavy, farting is a real danger. (I'm a massive curry eater and ale drinker. yoga studio's ECHO lol)
Everybody is going to be arse up at some point so, make sure your scrupulously clean as smelly feet and sweaty balls will not make you friends.