On top of your tablet device, there is a button to the left which is both your lock and power button. To lock your device, press the button once.
To unlock your device, press the button again. It will then show you your locked home screen. Swipe upwards to unlock. If you have a security pin, you will need to enter this to unlock your device.
To turn the power off or restart your device, long press the button and two options will appear to the right of your screen. If you choose to restart, your device will automatically restart itself. If you choose to power off, you will need to long press the power button to turn it back on.