OS X Yosemite: How to Set the Login for Kiosk Mode
Planning on setting up a public access (often called a computer kiosk) Mac? You can change the Automatic Login account from the Users & Groups Preferences pane in System Preferences to configure your Mac for unattended public use.
Click the Login Options button under the User list, and then click the Automatic Login pop-up menu to choose the account that automatically logs in when OS X starts. Enter the account password in the confirmation sheet that appears and click OK.
Automatic Login is not a good security feature in many cases (as with a laptop on the road), but it can be A Good Feature for those preparing a Mac for public use — if you set the Automatic Login account to your public standard-access account, OS X automatically uses the correct account if the Mac is rebooted or restarted.
It’s always a good idea to create a Managed account for use in kiosk mode — you can assign the Simple Finder to the account, and even specify which applications and system functions that public users can access.
You can always choose Log Out from the Apple menu and then log in under your own account.