If you do not have screen off function keys in your keyboard then you can use following commands to turn your screen off.

First you need to find out your screen identifier:

$ xrandr --listmonitors

Monitors: 1
 0: +*LVDS1 1600/310x900/174+0+0 LVDS1

Then turn screen off with the following cmd:

xrandr --output LVDS1 --off

This will totally turn your screen off.