A citation should be used when content that did not originate with you is used to support your writing.
Here are a few resources that will help get you started:
No need to cite what is known as common knowledge.
But, what is common knowledge? Common knowledge includes information that someone can easily trace and find the answer to. In an academic context, common knowledge will depend on the audience, but here are some examples to get you started:
For more examples of how to determine if something is common knowledge, check out this resource:
When in doubt: CITE -- You won’t be judged harshly for adding a citation when it isn’t needed. However, skipping a citation (whether it’s an accident or not) is considered plagiarism, which can have serious consequences.