Addressing a letter has more to it than writing a name and a mailing address. Or just writing a name at the top of a note with a comma behind it. Many subtleties and formalities should be taken into account. They are based primarily on the letter writer's relationship with whoever is receiving the letter. There is also the aspect of where the letter is going and whether it should be addressed to a particular person, a business or a group of people. Below are the rules of thumb for properly addressing letters.
First, who is this letter being written to? Is it a close friend? If so, you can write just about anything, including a cutesy nickname or their initials. This conveys a little extra connection to the person who is receiving the letter in the first place. However, if it's someone you don't know well or at all, addressing a message formally is paramount. Use the titles of Dr. / Mr. / Mrs. or Ms. before their name and maybe even a professional title of some kind after.