As a rule of thumb, these are three basic elements, but this will vary with the type of source:
WHO/WHAT: Identify the element of the source (author or title) which provides the greatest authority and/or secondary credibility. Does the author have credentials? What type of publication is it—newspaper, government report, magazine, journal? (In other words, would everybody know that the Kansas City Star is a newspaper? If not, tell them!)
WHEN: When was the book, magazine, newspaper or journal published (date)? When was the person interviewed? When was the website last updated and/or when did you access the website?