Whenever you quote or base your ideas on another person's work, you must document the source you used. Even when you do not quote directly from another work, if reading that source contributed to the ideas presented in your paper, you must give the authors proper credit.
Citations allow readers to:
Think of documenting your sources as providing a trail for your reader to follow to see the research you performed and discover what led you to your original contribution.
Failing to do so constitutes plagarism, which comes with serious academic consequences. As you gather sources, make sure you have all the information you need to cite a source, even if you're not 100% sure you'll use the source.
View this guide, Giving Credit Where Credit is Due, to learn more about why and how to cite sources.