What do do with a nippy horse?

So later on I'm going for a riding lesson, on a 15hh mare called Millie. She's a very sweet girl but is very old and has a sensitive mouth, knows all the tricks of the trade, how to fool inexperienced riders and all that jazz. My friend, who also rides at Franklin's, told me that Millie likes to bite people. I thought that if a horse misbehaved, a tap on the shoulder with a crop would normally put them back in their place, but my friend is now saying that if you hit horses, they'll just bite you back, and harder. What can I do so that she won't bite me? And if she does, how can I get her to stop?