For general info, such as how to navigate the operating system, personalize the Start screen, utilize apps and customize settings, refer to the Windows 8.1 & Windows RT tutorial.
Lock the Device
From the Start screen, tap the user account picture located in the upper-right. To navigate to the Start screen, swipe left from the right-edge of the screen (to display the charms menu) then tap Start.
Unlock the Device
If necessary, press the Power button located on the top-edge (left of the volume keys) to wake the device.
If necessary, swipe up to dismiss the lock screen.
If necessary, select the appropriate user account.