Newandworried,
Don't be worried, nothing bad will happen tomorrow, If you are a car driver, you have a far greater chance of being taken out on the roads.
It's natural to be apprehensive, but don't let it take over. Relax and enjoy the experience, there is nothing quite like it.
Your instructor will brief you on what to expect, and will talk you through whats happening during the lesson. Be aware that it is highly addictive and you will replay the whole experience over in your mind for days afterwards.
Good luck, and have fun.
T18