OS X Yosemite: Enable FileVault 2

With FileVault 2 you can encrypt the contents of your entire drive to help keep your data secure. FileVault 2 uses full disk, XTS-AES 128 encryption to help keep your data secure. Using FileVault 2, you can encrypt the contents of your entire drive.

System Preferences > Security & Privacy > FileVault tab

(click the lock to make changes if necessary)
Mac FileVault 2 01
Choose your recovery method.
Mac FileVault 2 02
Restart your computer.
Mac FileVault 2 03