The key to avoiding plagiarism is to make sure you give credit where it is due. This may be credit for something somebody said, wrote, emailed,
drew, or implied. Many professional organizations, including the Modern Language Association (MLA) and the American Psychological Association (APA), have lengthy guidelines for citing sources. However, students are often so busy trying to learn the rules of MLA format and style or APA format and style that they sometimes forget exactly what needs to be credited. Here is a brief list of what needs to be credited or documented:
Bottom line, document any words, ideas, or other productions that originate somewhere outside of you.
There are certain things that do not need documentation or credit, including:
When you are using generally-accepted facts (e.g., pollution is bad for the environment) including facts that are accepted within particular discourse communities (e.g., in the field of composition studies, "writing is a process" is a generally-accepted fact).
Generally speaking, you can regard something as common knowledge if you find the same information undocumented in at least five credible sources. Additionally, it might be common knowledge if you think the information you're presenting is something your readers will already know, or something that a person could easily find in general reference sources.
But when in doubt, cite; if the citation turns out to be unnecessary, your teacher or editor will tell you.