You can find the name and address of your representative by going to Congress.org. When addressing your letter, use the following format:
For Your Senator:
The Honorable (full name)
(Room Number) (Name) Senate Office Building
United States Senate
Washington, D.C. 20510
Dear Senator (last name):
For Your House Representative:
The Honorable (full name)
(Room Number) (Name) House Office Building
United States House of Representatives
Washington, D.C. 20515
Dear Representative (last name):