My only two cents to put in is this:

Just like any other animal, i you go in scared they will know. Just be positive and you will have a fabulous time!! I have been riding since I was 5, and have loved every moment! It is such an awsome thing to do. I am sure your moms friend will put you on a horse, that is quiet, and used to inexperianced riders. And I also have never heard of a horse rolling on anyone!