If you do not know the name of the person you are writing to, begin with Dear Sir or Dear Sir or Madam or Dear Madam and end your letter with Yours faithfully, followed by your full name and designation.
How do you write a good letter to a stranger?
Short and sweet is great. Writing a letter to a stranger is an act of kindness: make sure your letter is not about you, but really lifts up the other. Don't outright ask them to write back, or write in a way that compels them to reply. This puts pressure on an act that should only bring joy.