Wanabe, there is no reason why you can't spend a few lessons just in walk until you get some confidence. While walking you can practice your position and balance and turns and things like that. As for sitting trot, mostly you just start with a few strides of sitting at a time until you get used to it.
If you are fit and strong, you shouldn't have any problem- as Wally said, if you are on a beginner's horse, the horse will take care of you and you won't be going fast enough to have to worry about reaction time.
I was actually AFRAID of the horse when I first started 2 years ago. When I think about her now, i realize that she was a very old, very gentle horse who was the sweetest thing ever and never put a hoof wrong.
Hurry up and start lessons so we can hear all about it.
As for your questions in your other posts, try getting a basic riding book out of the library or at a bookstore and it will tell you a lot or look at the articles on the New Rider home page. ( I'm not saying not to ask them on here though!)
Good luck!