Consult the "Publication Manual of the American Psychological Association," which bases its law-related citation guidance on standards set forth in "The Bluebook: A Uniform System of Citation." As Chelsea Lee explains at the APA Style blog, if the "Publication Manual" does not provide information specific to a source you need to cite, go to "The Bluebook."
Cite Supreme Court cases, for example, in the text of your paper using the following format, adapted from Westfield State College's library website. Be sure to underline the name of the case, as per "Bluebook" style.
The Supreme Court made XYZ the law of the land in United States v. Jones (2006).
Create a reference list citation as follows. The "380" refers to the volume number of the case, while the "192" refers to the page number.
United States v. Jones, 380 U.S. 192 (2006).