In any discipline, citing your sources is critical to supporting your argument but perhaps this is nowhere more true than in law, where precedent is often used as the basis for legislation and court decisions. Legal citation follows its own rules, so here are a few helpful sources to help you cite legal sources properly.
Cornell's Guide to Legal CitationA fairly comprehensive guide to citing judicial opinions, laws, constitutions, rulings and more. Includes video tutorials.
The Indigo BookThe Indigo Book is a free, Creative Commons-dedicated implementation of Harvard Law Review Association's Bluebook Uniform System of Citation.