Address the letter to "Mr. King," not "Stephen" or "Steve." Maintain respect.
Tell him which of his works you like best and why. Keep the letter as short as possible. He probably can't read longer letters. Mention how the books might have influenced you, but refrain from getting too personal.
Send only fan mail. At one time, fans could mail a request to have up to two books signed by King and, once they received a note directing them to do so, could send in the books for autographs. But King now only autographs books at signing events.
Realize that King might not even read the letter. King has an office and a secretary who handles all of his day-to-day business, including going through his mail.
Mail only to his office address or his post office box. You can find both addresses online. King strongly discourages fan mail being sent directly to his house. Again, respect the author's wishes.