Lock keys and screen
Want to avoid accidentally making a call when your phone is in your pocket or bag?
To lock the keys and screen of your phone, press the power key.
Unlock the keys and screen 
Press the power key, and drag the lock screen up.
Set the keys and screen to lock automatically
Tap 
Settings > lock+wallpaper > Screen times out after , and select the length of time after which the keys and screen are locked automatically.