I remember my first experience in front of a class... I said 'OK' and 'So...' far too much, I shook like a rabbit in the headlights and I missed out half of my lesson plan!
And I came from a background in corporate software training!
You have to take the plunge and get the first one over with, then it's much, much easier!
The one thing I would caution is that being a teacher you are very much in the spot light and that never completely goes away. I have to deliver demo classes for prospective new students and their parents, I have to do 'Open door' lessons where the parents of my regular classes come in to watch; the performance aspect is always there. If it is something you are genuinely scared of, then maybe teaching ESL isn't the right thing for you. Most people teach in a different country, where they have to get used to a new culture, new way of life, language and a new job all on their own. If you're scared of being in front of a class, these adjustments can seem terrifying.
2 years ago a woman from Canada came out to China to teach at the school I worked at. She came in to teaching because she'd always thought about doing it and she figured she'd give it a try. She had such stage fright in the classroom she could barely speak and ran out of most of her classes during her first week. She didn't have a very calming or commanding presence and she lasted two weeks before packing her bags and returning to Canada.
So long as you can take control of your nerves and concentrate on what you're doing you should be fine. If anything, teach adults and use the age old stage trick of picturing them in their pants... This doesn't work if you want to teach children, for obvious reasons!