Citing your sources can seem time consuming when writing a paper but there are important reasons to cite your sources. Most importantly, citing your sources helps you avoid plagiarism, which is presenting someone's ideas as if they were your own.
Citing your sources shows your instructor that only did you research your topic using appropriate resources but it also shows that you have investigated multiple viewpoints and drawn your own conclusions.
Citations allow your reader to find your research sources. Just as you used bibliographies cited in books and articles you used to write your paper, you reader could follow your citations for further research on your topic.
Scholars and students who do research must cite their sources. It demonstrates that they are part of the scholarly community, responding to current research either by agreeing or disagreeing and by adding to the research with their own conclusions and ideas.
Other people's words, ideas, and other original work (photos, lyrics, poetry, music, etc.) that you use to write your paper must be cited.The following are examples but are not an exhaustive list.
You do not need to cite common knowledge: facts and concepts that are widely known. Examples of this are things like "Abraham Lincoln was the 16th president." or "Mexico City is the capital of Mexico."
If however, you cite something that contradicts what is commonly known then you must cite your source. An example of this would be if you stated that John Lennon was killed by the CIA who used mind control on Mark Chapman. That would need a citation because it is not generally accepted as fact.