A good way to develop topics, brainstorm search terms, or learn context about a broad subject is to use resources like the Oxford Research Encyclopedia of American History. It will point you at critical works and help you discover scholarship in topical areas.
There are many handbooks, dictionaries, and encyclopedias that provide basic information for historical studies. If you are selecting a topic, build a framework by consulting definitions or a quick overview found in these resources and then focus on your topic by searching for more specific information. Find these materials online (sources to use to get started are in the box below) and in the library stacks (Call number "D" = general and "F" = American history.)