The record center is a huge place and your request for action is going to take months as it is. Don't create extra time for yourself by sending a letter that might wander around a number of desks before it reaches someone who can actually act on it. There is always a huge backlog of requests, so expect a long wait. No special form is required, but be sure to include the following information neatly and prominently in your letter:
Your full name
Social Security number
Service number (if you served before 1970)
Date and place of birth
Approximate dates of service
NOTE:The report of separation form issued in most recent years is the DD Form 214, Certificate of Release or Discharge from Active Duty. Before January 1, 1950, several similar forms were used by the military services, including the WD AGO 53, WD AGO 55, WD AGO 53-55, NAVPERS 553, NAVMC 78PD, and the NAVCG 553."