Sources can generally be classified into two categories: popular and scholarly.
Popular sources are typically written for a general audience. Though they may be edited for clarity and style, popular sources are not vetted by experts in the field like scholarly sources. They can be useful for popular topics though!
Scholarly sources are written for a scholarly or academic audience. Some scholarly sources are also peer-reviewed, meaning that the research has been vetted by other experts in the field prior to publication. Some indications that a source is scholarly are:
Evaluating sources can be challenging! The questions below are a good starting point for thinking through an information source and judging its credibility and utility for your research needs. See if you can answer "yes" without reservations to the following questions and, if you can't, try to dig a little deeper into your source or ask a librarian for help.
Don't discount all popular sources outright. Although you will usually need scholarly materials for most of your research, popular sources can be helpful depending on your topic and your needs. Popular sources can be great for...
Even if you are using popular sources, make sure you are thinking about your sources critically. Whatever information you use, you should still think about who wrote it, what biases they may be bringing to their treatment of the information, and whether they are citing their information sources.
The following pages from the center for Teaching, Learning, and Academic Support are great resources to help you evaluate your sources!
And remember that if you can always reach out to a librarian if you're still in doubt!