Hi minud,
I don't quite understand what you are trying to ask, but perhaps this will help you.
When writing an official letter to a person whose name you do not know, your salutation should be: Dear Sir/Madam.
When writing an official letter to a person whose name you do know, your salutation should be: Dear Mr. X. You should replace "X" with the person's surname, and "Mr." with their correct title - which may be Mr, Mrs, Miss, Ms, Dr and so on.
When writing an official letter to a person whose name you do not know, your valediction (how you end the letter) should be: Yours faithfully, followed by your name on the next line.
When writing an official letter to a person whose name you do know, your valediction (how you end the letter) should be: Yours sincerely, followed by your name on the next line.