This is the weapon-training portion of the tutorial.  To pick up a weapon just put your cursor over it and press the "Use" key (defaults to "Enter").  To fire the weapon you need to select it.  Do this by pressing the "Next Item" key (defaults to Page down) then the "Fire" key, or use your mouse wheel to select the weapon then press "Fire".  To drop a weapon you have picked up select it and press the "Drop Item" key (defaults to "D").

This is the machinegun section of the weapons training area. Machineguns are not very accurate, but they spray a lot of lead the enemy's way.  See those little red lines around your crosshairs?  The further those are away from the crosshair, the less accurate your shots will be, meaning bullets will not hit what is directly under the crosshair.  Play with these guns a bit to see what makes the red lines diverge.  Learning how to accurately fire machineguns is a key to victory in Line of Sight.

After you are done move to the next Training Area sign.