Follow your style guide.
When you see the instruction to "use MLA style," for instance, that means your paper itself as well as your citations need to look a certain way. (There are even rules documenting when to write out numbers or preferred phrasing and things like that, but we won't get into that here.)
If you don't see a style guide named in your assignment, ask your professor what they'd prefer! MLA is the most frequently used in our classes, but it depends on the subject! Psychology and speech classes frequently prefer APA, while history and art history may have you use Chicago Style.
Citations come in two parts.
- Short, "in-text" citation: Apply this whenever you've used a source in your paper. It should connect easily to the full citation. The style guides have rules for what these should look like.
- Full detailed citation: all the details to help someone locate your source, arranged in a certain order according to the style guide, and listed at the end of the document.
Don't plagiarize; give credit.
When do you need to insert an in-text citation? Every time you use information not original to you. "Quotes" are a major red flag pointing out where citations need to appear, but even if you paraphrase, you need to give credit! It's about ideas, not just language. When you quote, you're borrowing ideas and words; paraphrase, you're borrowing just the ideas... but you are still borrowing something from another creator.
Exceptions:
- Commonly-known facts. The sky is blue, the capital of Texas is Austin, a simile is a literary device that likens one thing to another... these are pretty obvious. What counts as a commonly-known fact depends also on who the audience of your paper is supposed to be (and therefore their familiarity with the subject). That is, you'd explain more about the elements of art in a paper for engineering majors than you would art class; in class, jump right into applying those elements.
- You can make some compromises for readability. If you have several sentences in a row with the same (Smith) citation at the end of each, you can wait until the last sentence to include your in-text citation.