There are many different citation styles. The library provides online guides to the most common styles.
Which style should you cite your sources in? Ask your professor! Different professors prefer different styles.
If your professor doesn't have a preferred style, a good choice for History or American Studies classes is Chicago Note-Bibliography style, which is the standard style in these disciplines.
Chicago Note-Bibliography style requires two types of citations: footnotes (or endnotes) and a bibliography at the end. Here is an example of an entire paper in Chicago style, courtesy of the Online Writing Lab at Purdue University:
Zotero is a free app that helps you keep track of the sources you've found, and lets you generate citations automatically. It's available for Windows, Mac, and Linux.
If you don't want to download an app, go to zbib.org to use ZoteroBib, a simplified version of Zotero.
Caution: If you use an app or website to generate citations automatically, be sure to proofread the citations before you submit them to your professor. Automatically-generated citations can contain errors.