When you speak to royalty, there are specific ways you should address them. According to the royal family's website, it's easy to remember: upon first addressing the royal, use either "Your Majesty" (for the queen) or "Your Royal Highness" (for any other HRH-titled royals such as Prince Charles, the Duke and Duchess of Cambridge, and so on). Upon subsequent addresses, just "Ma'am" or "Sir" will do.