Your first action will be to secure basic necessities for day-to-day survival. Water, food, medicine ... and weapons. Lots and lots of weapons. The next step will be to flee heavily populated areas because where there are people, there are souls desperate enough to do anything to stay alive. Your third phase will be to find a refuge that protects you from the wandering hordes — hordes of the undead. Legions of zombies, all scrambling to eat any humans left over from a ruined civilization.
Zombies have been a fixture of folklore and creative media for hundreds of years, but they've really sprung (or lumbered, if you prefer) into the limelight in the past decade or so. There have been a number of different types of zombies theorized by writers and scientists. Some are caused by a virus that infiltrates and manipulates the human body. Others are a result of radiation exposure. Still others are a manifestation of a voodoo curse or perhaps a parasitic fungal infection.
Zombies from 1968's "The Night of the Living Dead" were actually called ghouls, but they definitely exhibited many of the virtues of what we consider zombies. They slowly but relentlessly clawed their way toward any breathing person they could find, making up for their lack of speed with ceaseless patience and overwhelming numbers.
More modern zombies, such as those from 2013's "World War Z" might be a reflection of our faster-paced, Internet-fueled societies. They're undead, yet they're also capable of running down slow-footed victims, and they exhibit flickers of intelligence, too.
Zombies are a physical paradox. They're undead, yet they move around like they're alive. They're cold and lifeless, but somehow they crack open skulls to dig out a dessert of brains. They're rotting away but also stumbling down city streets grabbing unlucky people left and right.
Luckily for you, according to our current understanding of human biology, zombies just can't happen.