Dear (insert last name unless writing to someone you know well),
Paragraph 1: The content for your first paragraph obviously depends on the subject matter at hand, but a good rule of thumb is to remember to explain who you are and why you are composing a letter to this person. Of course if this letter is to someone you know well, then you can be a bit more informal with the structure of the letter.
Paragraph 2: This paragraph should revisit why you are writing, and expand upon the subject. Explain the reason for correspondence fully, and state what you hope to achieve through the letter, whether that be an acknowledgement of some sort or a response to a specific question.
Paragraph 3: This paragraph should wrap things up. Restate the reason for writing, and thank the reader for their time.
Sincerely, (Kind Regards can be used if you are in regular contact with this person)