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).
The general format for Blue Book citations include a name, a series abbreviation, and a year. The series abbreviation usually appears as a volume number followed by the abbreviated name of the series and then another number, often a reference to the first page.
Arizona v. California et al., 547 U.S. 150 (2006), is a citation to a case involving Arizona and California, which can be found in volume 547 of the United States Reports (U.S.) beginning on page 150 and which was decided in 2006.
Probably the most comprehensive, free online resource for legal citation is Peter W. Martin's Basic Legal Citation, 2013.