A citation should be used when content that did not originate with you is used to support your writing.
Content includes:
The UW Tacoma TLC Writing Center can help you understand proper citation within your work.
Don't cite common knowledge.
Cite ideas that critique "common knowledge," new ideas within a field of study, opinions, interpretations, and study results.
For more examples of how to determine if something is common knowledge, check out The Exception: Common Knowledge.
You won’t be judged harshly for adding unneeded citations. On the other hand, skipping a citation--accidentally or not--is considered plagiarism.