Your instructor may want you to use scholarly (or "peer-reviewed") sources. What does this mean?
There are two main types of scholarly sources:
Articles published in scholarly journals (print or electronic), which are usually peer-reviewed.
Books (print or electronic) intended for an expert or specialized audience.
Scholarly sources are:
Specialized: written by scholars for an informed, academic audience, at a level that requires some background knowledge in the subject
Build upon the work of other scholars, often including extensive bibliographies.
Examples: Proceedings of the National Academy of Sciences, Journal of African American History, and JAMA (Journal of the American Medical Association)
Popular sources, on the other hand, are intended for the general public. These sources are more introductory, may not be written by experts in a field, and often do not cite any other sources. Examples of popular magazines include National Geographic, The Economist, Time, Newsweek, and People.
How can you tell if an article or book is scholarly? Look for:
Who wrote it? Does the author have an advanced degree or a university affiliation?
Who is the audience? Is the writing style introductory or advanced?
Who published it? If your source is an article, what journal was it published in? Is that journal a peer-reviewed source? If your source is a book, was it published by a university press or an academic publisher, like Sage or Routledge? (Note: Publication by a university/academic press is not a guarantee that a book is scholarly -- just a good sign.)
Footnotes/endnotes/bibliography? Does your source include a bibliography and/or footnotes with citations of sources used?
Other features of the 'best' research:
Peer-reviewed (applies mainly to articles from journals): Before publishing, the article was vetted by other scholars in the field.
Has been cited by other scholars. (This can take time, so the newest articles might not be heavily cited yet.) How do you know if your article has been cited by others? Try searching the article citation in Web of Science or Google Scholar, which indicate the number of other articles that have cited your article. (Note:Neither of these sources is comprehensive.)
Scholarship is always changing. Try to find the most recent scholarly sources you can.