Secondary sources are the scholarly analyses of primary sources.
Searching for secondary sources begins with two or three "Keywords" that best describe your topic.
Keyword searching often begins, not surprisingly, in Google, whose combination of algorithms and use data provides results with ever-improving relevancy. However, since it is important to verify the scholarly credibility of any source when doing academic research, it is best to search in Google Scholar, a subset of Google which returns results only from scholarly literature.