To yank one line, position the cursor anywhere on the line and type yy. Now move the cursor to the line above where you want the yanked line to be put (copied), and type p. A copy of the yanked line will appear in a new line below the cursor.
To place the yanked line in a new line above the cursor, type P.
The yy command works well with a count: to yank 11 lines, for example, just type 11yy. Eleven lines, counting down from the cursor, will be yanked, and vi indicates this with a message at the bottom of the screen: 11 lines yanked.