Combat

Combat is composed of two parts: Surviving against enemy attacks and killing the zombies.

Surviving
Mobility is key. Keep the attackers at range and focus on taking them quickly and accurately. Above all, don't get crowded into a dead end. That's, as the name suggests, a poor tactical choice. Always make sure you know whether or not you can get away if you have to, and assume that every single room you're entering might have a surprise attack. If you see any doors or toilet stalls in the background of the room, assume that they are hiding ambushes and prepare accordingly. Zombies also rely on line of sight to spot and target you, so if you get out of visual range, they'll immediately forget you're there.

Killing
As a living human against an undead horde, you have the advantage of working fine motor skills. Apart from shooting the zombies in the head with your weapons, you can also:
 * Punch the zombies. Build up your punch by holding the right mouse button for a mostly one-hit kill, or punch once to knock off armor. This is essential to saving ammunition because you will want it in situations where melee won't save you.
 * Rip off elements of the scenery and throw them at the attacking hordes. Smacking down a zombie with a toilet is possible - and encouraged.
 * Just shooting elements of the scenery, like petrol barrels, make a huge explosion and kill them - remember keep at range and not hurt yourself.
 * Pick them off from a distance. If you are far enough away, your shots will be out of hearing range of other zombies further away (as weird as that seems given how loud weapons are, but no point in looking a gift horse in the mouth).
 * Plant traps. If you see a petrol barrel sitting nearby, the game generally intends for you to use it soon. Place the barrel down next to the door with the right mouse button, then light it and immediately pop the door open, then run for the hills before everything gets shredded.