Dear (insert contact and address by title or last name),
Paragraph 1: This should be an introduction to your letter and you can also introduce yourself in this section if the reader is unlikely to know you.
Paragraph 2: Here you should go into detail about your subject matter. Take the time to explain why you are writing and indicate what you hope to achieve through your letter.
Paragraph 3: Here you can reiterate what it is you are trying to say through your letter, and hopefully include a call to action for the person who is reading the letter. Sign off by thanking the reader for his or her time.