We must cite sources to only only give credit where credit is due, but to point our readers to find the sources that we use in our papers, speeches, or presentations.
There are several types of citation styles, and your major/discipline will often determine which one you use, but your professor will also specify if they want you to use a certain style.
Two of the most common ones are APA (from the American Psychological Association) and MLA (from the Modern Language Association).