The MLA Style and Formatting Guide from the OWL at Purdue is a great resource to consult about citations and formatting for your paper.
Most library databases, such as the library catalog and all Ebsco databases, have a feature that will show you how to cite the article or book you are viewing in various citation formats. Look for a "cite" or similar link.
If you use the words or ideas of someone else, you must give proper credit. If you don't, it is plagiarism. Plagiarism is part of academic dishonesty and can have serious consequences.