A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007

Read tertiary sources for introductory overviews
Finding useful sources
Part I. Research and writing: from planning to production

Tertiary sources are based on secondary sources, usually written for non-specialists. They include general encyclopedias and dictionaries, as well as newspapers and magazines like Time and the Atlantic Monthly and commercial books written for a general audience. Well-edited general encyclopedias offer a quick overview of many topics. Beware, however, of online encyclopedias, such as Wikipedia, that rely on anonymous contributions rather than on carefully edited entries written by established researchers. Wikipedia has proved to be relatively accurate in the sciences, but overall it is uneven and sometimes wrong. Never cite it as an authoritative source.

Be similarly cautious about using magazine and newspaper articles. Some describe research reported in secondary sources reliably, but most oversimplify, or worse, misreport it. You would, of course, treat such a source as primary if you were studying how it deals with a topic, such as gender bias in the Encyclopedia Britannica or hoaxes in Wikipedia.

Once you understand kinds of sources, you can begin looking for them.