Ethical use of information includes citing your sources. Even when you paraphrase (put into your own words), you should still cite where you found the idea. See Quoting, Paraphrasing and Summarizing for more information.
Online citation generators such as MyBib can help you with keeping track of your sources and creating a Works Cited List.
Writing the Works Cited list without a citation generator? Read more about MLA Guidelines for Writing & Formatting Your Works Cited List.