Primary sources can take many forms. Some examples:
Consult with your instructor if you're not sure what constitutes a primary source for your project. You can also consult this guide on primary sources.
Use the advanced version of the Libraries' Search by clicking "Advanced Search" under the search box on the libraries' website (libraries.usc.edu). Use the pull-down menu to choose "Subject terms," then type the title of a work you're interested in. This should give you books and articles about that work.
These works give background or introduction to a topic. The most common kind of this work is an encyclopedia entry, but they can also be books, websites, or articles.
Try a starting search for your topic in the main search box on the libraries' home page. Once you have some search results, use the "Material Type" limit on the left-hand side to limit to Books - then click on a title.
Note the call number, or book's location, at the bottom of the page. Try going to this call number in the book stacks of whatever library is indicated, and take a look at the books NEAR that book to discover some other angles on the same topic.