Know your product inside and out.
If you can't explain what exactly your business does and why it will be successful in one or two sentences, then you need to go back to the drawing board. First and foremost, you need to be clear and specific as to what the business is about. Once you've come up with something that makes sense, practice pitching your idea to your friends, family, even your mirror, until you can effectively answer any question that's thrown at you. It's the best way to work out the kinks.