Usually by their first name, and sign off with your first name.
In an informal letter to a close personal friend you may use: Dear Jane, Dear Janey, or Hi Jane,
In an informal letter to someone you know well enough to call by their given name, but not necessarily well enough to use a diminutive or nickname, use: Dear Jane, In an informal letter to someone you know only casually use Dear Ms. Doe or Dear Jane Doe, In most cases letters should be signed with your full name, think about it, how many people out there have the same given name as you... if there is any chance for doubt, use your full name.
Different ways to address a person:
what is the proper way to address the mayor
The proper way to address the conductor of a symphony orchestra is 'Maestro.'
the proper way to address the Queen is by "your majesty" or Ma'm, pronounced like "ham'
"Your honor"
usted. or ustedes.
Gentlemen
The proper way to address a Doctor of Osteopathic Medicine (DO) (not Doctor of Osteopathy) is Doctor (Dr.) since they are medical physicians.
Master John Doe
What is the proper way to address a meeting location in the Board Room of a building
The proper way to address a mailing envelope is to place the stamp at the top right corner, add the senders return address on the top left corner of the envelope with the recipients full name and address in the middle of the envelope.
The correct way to address a gift is "Mr. & Mrs. James Smith". The couple is already married so it is proper to address it this way.
Format for a business address: Person's Name and Title (if applicable, if known) Department (if known) ABC Company 1234 Main Street City, State 12345