To copy a case's citation, follow these steps:
Click on the "Copy Cite" (or "Copy Citation") button at the top of the case page.
Open up your draft document or email (where you want to paste the citation)
Paste (Ctrl+V) the copied cite. For the example below, the pasted citation is Connick v. Thompson, 563 U.S. 51 (2011)
If you want to copy text of a case in addition to the citation, visit this page to learn about copy with cite.