It's important to properly cite sources in a research paper or project so you can accurately give credit for information that is not common knowledge or an original idea. Proper citation will help you avoid plagarism, and is an essential part of the research process.
The State University of New York at Albany has provided a useful resource for understanding when, where, and why to provide citations.
SUNY's resource explains when you should provide citations:
While professors and scholars may have specific requirements based on the needs of their discipline, there are cases where you should always cite your sources.
A common question you might have about citations is what style to use? Different disciplines often use different citation styles, so you might use MLA citation styles for an English class, for example, and APA style for your psychology class. An overview of citation styles is here: https://en.wikipedia.org/wiki/Citation#Styles
If you are uncertain which style you should use, always check with your professor.
Here is list of a few disciplines and the common styles used: