Primary Sources:
Primary sources are first-hand, authoritative accounts of an event, topic, or historical time period. They are typically produced at the time of the event by a person who experienced it, but can also be made later on in the form of personal memoirs or oral histories. Think original documents.
Secondary Sources:
Secondary sources interpret or critique primary sources. They often include an analysis of the event that was discussed or featured in the primary source.
Try a simple search for "League of Nations" or the "Woodrow Wilson" to find some excellent secondary sources! Using secondary sources for background research is optional.