Most popular search tools:
RefWorks - will format your citations into any of over 3,000 styles and help you generate your list of references
ZoteroBib is a very quick way to get a few citations formatted in APA, MLA, or one of many other styles, no account setup needed.
Also check the subject guide for your department for links and tips specific to your field.
EBSCO (including OneSearch), Proquest, Ebrary, and EBL all include features to give you the citation for a particular article or ebook within their search interface. Many other publisher sites (ScienceDirect, Wiley, etc.) may also have this feature.
Ask a library staff member where to find these. Their citations may not be perfectly correct (especially with regard to capitalization) so check what they give you against the official rules.
APA (American Psychological Association) 7th edition (new Oct 2019!):
APA (American Psychological Association) 6th edition:
MLA (Modern Language Association):
MLA 8th Edition (released spring 2016):
MLA 7th Edition:
Chicago/Turabian, 16th edition:
Other useful links: