Always do reading in secondary sources before looking for primary sources. Often times, information in secondary sources will lead you to a primary source. For example, an article in a reference book might reference a speech or letter written by the person you are researching.
Primary Source vs. Secondary Source
A primary source is a document that shows direct, immediate or firsthand knowledge of a subject or event. It is a document written at the time or on the scene where an event occurred.
Secondary sources are those writings without direct knowledge of a topic or event. These include biographies, monographs, and general periodical articles. Secondary sources are written by people who did not witness or experience an event but have a great deal of knowledge about the topic. Writers often use primary sources in their writing of secondary sources!