Your instructor allows you to choose from MLA, APA, or Chicago formats. You should choose one format and be consistent in using this format.
Below are citation guides from Penn State University Libraries, as well as the Purdue Online Writing Lab (Purdue OWL) which will be helpful in providing examples and guidance for creating your citations.
You can also find the citation manuals in book form at the Library.
NOTE: Many databases will provide pre-formatted citations with articles. It is in your best interest to verify the formatting of these citations before copying and pasting them into your paper. While the information in the citation is generally correct, the formatting will more often than not be somewhat incorrect.
University Libraries Citation Guides
Purdue OWL Guides
Citation Manuals at the Library