In this case, you should start your letter with "Dear " followed by their last name and an initial for their title. For example, "Dear Mr. Jones" or "Dear Dr. Smith." Be sure to include your name along with theirs on the letter so they know who it is from. You can also sign your letter with a line indicating who you are. It's important to note that you shouldn't use first names unless the person tells you otherwise. This is showing lack of respect.