Skip another line before the salutation, which should be followed by a colon. Then write the body of your letter as illustrated here, with no indentation at the beginnings of paragraphs. Skip lines between paragraphs.
After writing the body of the letter, type the closing, followed by a comma, leave 3 blank lines, then type your name and title (if applicable), all flush left. Sign the letter in the blank space above your typed name. Now doesn't that look professional?
Sincerely,
John Doe Administrative Assistant