So if its your first time having a gun, you better know the basics, left mouse button is to shoot and right mouse button is to aim.

When you click RMB, you aim with your sights, you can modify your weapons and sights at any workstation, but a weapon manager tablet is required.

To actually aim at all you need to do is to hold RMB and shoot the enemy in the head. So by now you already have enough knowledge to pretty much kill anybody with no issues.