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.
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.