You see only your part of the map, but the Visor allows to widen the borders of the area that you can see. It is dark out there, and you won't see supplies, upgrades and enemies until you get close!
You start shooting automatically when an enemy enters the attack area (the cone of red light from your weapon). The attack area depends on the weapon of your hero.
Walls and objects stop the bullets unless your weapon allows shooting through the walls. The piercing power parameter is relevant for such characters as Levi, Slayer, Bertha, and Leviathan. This parameter determines whether the hero's shots can pierce obstacles such as walls or energy shields.
Obstacles such as crates and doorways allow shooting from cover if you peek out just enough for the enemy to be in your attack area.
When you move, your aiming is worse. Moving widens your attack area, and your shots may miss their goal. Stop to aim better, and your shots will become more precise.
Shooting is loud, and other players nearby can hear it. They can also see the bullet trajectories if they are close and Shooting icons in the direction of the shooting if they are far away. Take this into account if you don't want your location to be known!