A legal citation is a reference to a legal document such as a case, statute, law review article, etc. 
First, you should check the documentation for the citation system you are required to use, e.g. APA, MLA, etc. Here's a page about citing legal sources in APA, for example.
Sometimes these citation systems (or your professor) will direct you instead to the standard legal citation system from The Bluebook: A Uniform System of Citation (available only in paper for Ohio University affiliates).
Probably the most comprehensive, free online resource for legal citation is Peter W. Martin's Basic Legal Citation, 2013.
For Chicago Style legal citation, check out the Owl Purdue website for more information.