Dear Sir, Madam, (If you do not know the name of the person)
Dear Mr / Mrs / Ms Peterson, ( If you do know the name of the person)
Letter:
Divide in paragraphs: each topic has its own paragraph (paragraph 1 = Introduction; last paragraph = conclusion)
Finish with:
Yours faithfully If you start the letter with Dear Sir, Madam,
Yours Sincerely If you start the letter with Dear Mr Peterson