Your ultimate goal is to protect the air purifier at all costs. Zombies will be attacking you in waves. Make sure when you kill your zombies that you collect any money or ammo that can appear after a kill. When the level is over, use your money to upgrade weapons and fill your inventory as much as you can. Before a wave comes you can try and set traps to kill some of the zombies without wasting ammo. They will only kill one though and then they will disappear. Also, head shots will get you more money and some zombies can only be killed with a head shot.