Once you have found a source that is relevant to your topic, that source can lead you to other useful works!  This type of analysis is sometimes referred to as snowball searching.

There are two ways to analyze a source to find additional, related works:

1) Bibliographic analysis.  Look at the bibliography included at the end of an article or book chapter.  If you see an article of interest, use the library's Summon Search or Penn State's Google Scholar to look it up by title and find the full text of the article.  If you see a book of interest, look it up by title in the Libraries' catalog.

2) Citing works. Once an article or other work is published, it will be cited by other works.  If another work is citing a source relevant to your topic, chances are the newer work will be relevant as well!  To find citing works, look up the title of the original article in Penn State's Google Scholar or in Web of Science.  In Google Scholar, look for the 'cited by' link below the article listing.  Clicking on the 'cited by' link will show you works citing that work since its' publication, and is a great way to find more recent sources.

Note that some databases (such as Web of Science / ScienceDirect / ProQuest / PSU Summon) will also show you a link to 'related articles' / 'suggested sources' / 'documents with shared references' when looking at an individual article.  These articles may have a similar focus and/or cite similar sources to the original article and could be relevant to your topic as well.