Include:

• title or name of the case
• volume and page number of the court reporter, e.g. U.S.= United States Reports is the official source (reporter) of U.S. Supreme Court Opinions
• Date of the decision

In text:

Cite the name of the case (italicized) and the year of the decision.

• (Harper v. Virginia Board of Elections, 1966)

Reference List:

• Harper v. Virginia Board of Elections. 383 U.S. 663. 1966. URL

Explanation:

Supreme Court decisions are published in United States Reports, abbreviated "U.S." 383 U.S. 663 refers to volume 383 of United States Reports, page 663.

For additional examples, and for names/abbreviations for lower courts, see Cornell Legal Information Institute, Basic Legal Citation, Reporters & Courts.