good rule of thumb, is to address everyone as "My Lord" or "My Lady" (as suits the person you are addressing) - that is always polite and appropriate.
Some people you meet will have earned various other titles - such the right to be called "Lady X", "Sir Y" or "Mistress Z". People wearing crowns or coronets can generally be addressed as "your excellency". Follow the example of people around you - or ask the person you are speaking to how they would like to be addressed - but always feel free to use "my lord" or "my lady" if in doubt. A small bow or curtsey can also add an air of elegance, if you feel so inclined!