Most historians publish their work in books. So, for many topics in history, your best starting point is a good recent scholarly book. The easiest way to find that book is to ask someone else who is knowledgeable about your topic -- for example, your junior seminar instructor or spring JP advisor. But a thorough search of the library catalog is also essential. Here are some tips on finding books about historical topics in the Princeton University Library Main Catalog.
Once you have a book in hand, read it. Alas, there are no shortcuts to this part of the research process.