Plagiarism = copying an author's work and passing it off as your own.
To avoid plagiarism:
Information considered to be "common knowledge" does not need to be cited (unless you are directly quoting an author). If you are unsure, err on the side of caution and cite the source.
"Common knowledge" = anything that you expect the readers to know already
EXAMPLE #1
Common Knowledge (no citation or evidence needed): The sky is blue.
Not Common Knowledge: The physics of light refraction explains why the sky appears blue.
EXAMPLE #2
Common Knowledge (no citation or evidence needed): Ernest Hemingway wrote "The Sun Also Rises".
Not Common Knowledge: Hemingway's life experiences greatly impacted the story, characters and setting in "The Sun Also Rises".
The following websites may help you better understand and avoid plagiarism. Remember that there are serious consequences for plagiarizing.