"Primary sources are materials produced by people or groups directly involved in the event or topic under consideration, either as participants or witnesses." Secondary sources, on the other hand, "comment on and interpret primary sources".*
Hint: Another very good way to find primary sources is to consult the bibliographies of relevant secondary sources (books, articles, dissertations, etc.) that have used primary sources.
*Mary Lynn Rampolla, A Pocket Guide to Writing in History, 4th ed. (Boston: Bedford, 2004), pp. 5-6.