Grab your first item and try to make it out alive.
Eliminate the zombies from the rooms.
Get in the car befor time runs out.
Choose a weapon and fight your way through the floors, collect gold to buy upgrades and supplies, don't delay because the consequences will be dire.
If you spent the money you earned on the floors before leaving, you can go after the zombies outside, but be careful, they are tougher than they look.
Get the items you need to escape the area.