Encyclopedias and handbooks can be a great place to search for, and learn about, topics, gain background information, and find suggestions on where to look for further information.
The subject encyclopedias listed below are especially useful for establishing the context in which your selected person lived.
Start with biographical reference resources to establish the basic outline of and key moments in your person's life. Entries may include a discussion of the historiography on your person, that is, how scholars have debated and interpreted this person's actions and thoughts. Pay very close attention to the bibliographies at the end of each entry, as these will identify for you additional primary and secondary sources!
A good place to start is the American National Biography, and then branch out as needed.