The bible for general citation form is The Chicago Manual of Style. The Manual is available in many libraries, and subscribers can use the Chicago Manual of Style Online.
The bible for legal citation form is The Bluebook: A Uniform System of Citation, published by the Harvard Law Review Association. The Bluebook is also included in Prince's Bieber Dictionary of Legal Citations, discussed below.
The Bluebook can be confusing, but it is made somewhat clearer by Alan Dworsky's User's Guide to A Uniform System of Citation: The Cure for the Bluebook Blues. Other resources for understanding the Blue Book include Peter W. Martin's Introduction to Basic Legal Citation and Linda J. Barris' Understanding and Mastering The Bluebook: A Guide for Students and Practitioners (Carolina Academic Press). Caveat: If you are using a secondary source that is not based on the current edition of The Bluebook, check the current edition to make sure the rule in question has not changed.
Bieber's Dictionary of Legal Citations (William S. Hein & Co.) is another good source for accepted citation forms. Bieber's features a reverse dictionary that lets you look up the full names of materials in abbreviated citation format.
TACTICAL TIP: If you can't figure out an appropriate citation format, search for citations to the material in question in a relevant law review database on Lexis, Westlaw or HeinOnline. Whatever works there should work for you too.
Alternatives to the Bluebook: Several alternatives to the Bluebook have been published in recent decades. In 1986, the University of Chicago Manual of Citation ("The Maroon Book") was published but not widely adopted. In 1999, the American Association of Law Libraries came out with the Universal Citation Guide, with a second edition in 2005, and a third edition in 2014. In 2000, the Association of Legal Writing Directors produced the ALWD Citation Manual (Aspen Publishers).
For a critical comparison of the Bluebook and the ALWD Manual, see Has the Bluebook Met Its Match? The ALWD Citation Manual, 92(3) Law Library Journal 337 (2000).
Historical Case Reporters: To find out the name of the relevant reporter for a given court at a given time in the past, check out the Bluebook and/or the Bibliographical Index to the State Reports Prior to the National Reporter System.
State Citation Manuals: Some states have their own legal citation manuals, such as Texas Rules of Form, the Ohio Manual of Citations, and the New York Law Reports Style Manual, Official Edition (the "Tan Book"). Many of these are listed in The Bluebook (Bluepages Table 2, "Jurisdiction-Specific Citation Rules and Style Guides"). Otherwise, call a relevant state library, bar association library or law school library to find out if such a book exists for a particular state.
Universal Citations: In the 1990s a movement developed to avoid citations dependent on a particular publisher's version of cases and other primary legal materials ("vendor-neutral citations"). The Universal Citation Guide was designed specifically for this purpose. The Bluebook and the ALWD Citation Manual permit the use of Universal citations. A handful of states have adopted vendor-neutral citation; for the story up to 2007, see Neutral Citation, Court Web Sites, and
Access to Authoritative Case Law, 99 Law Library Journal 329 (Spring 2007), by Peter W. Martin. Arkansas adopted neutral citation in 2009 (see Arkansas Rule 5-2, "Opinions"), and Illinois followed in 2011 (Illinois Supreme Court Rule 6, "Citations"). See also "Universal Citation and the American Association of Law Libraries," 103(3) Law Library Journal 331 (Summer 2011).