If you're just starting your research, and have no experience with writing and researching in history, try using the Credo Reference database. See the search box on this page. Credo Reference is a general reference resource. Its full-text content covers every major subject, including history.
Specialized encyclopedias devoted to American History are often an overlooked resource for the researcher who is in the earliest stages of a research project.
Some encyclopedias are devoted to particular aspects or eras of United States History, for example: