Using someone else's words or ideas and passing them off as your own work is plagiarism, which is unethical and cause for academic consequences, including failing your class. The video below is from the WMU Libraries Instruct program, and explains in more detail what plagiarism is and how to avoid it. If you are logging in from off campus, you may need to authenticate with your BroncoNet ID and password to watch the video.
Your instructor usually determines which style you must use in your papers. If you aren't sure, ask your professor.
This page links to information on MLA style and others.
Quick Citation Formatting in Databases.