Purdue Online Writing Lab (OWL) MLA Style Guide - This website includes more in-depth help on how to cite sources in MLA style, as well as how to properly format other parts of your paper (the title page, appendices, etc.).
MLA Handbook for Writers of Research Papers - The official print publication manual for MLA style. If you can't find the answer to your citation question in one of the above style guides, check here. They also have some portion of this content online at the MLA Style Center.