Typically zombies are created when another zombie bites a normal person. Their virus transforms the body and you become one of the undead.
Depending on what type of zombie attacks you, will depend on how long it will take for you to transform.
There are some cases where when a person dies, they become trapped between death and the real world. They wander the earth in zombie form seeking out human flesh and brains. As the years have gone by the transformations and ways zombies are created have expanded.
In the past, it was always thought that zombies were unintelligent, and slow moving beings. With the ever-changing world we live in, where things are bigger, stronger, faster and better, these aspects are true with regard to zombie creation. There has been discussion that there has been testing done to create zombies to be used as a military weapon. Creation of mass armies could be utilized and dispatched in the event of war or other crisis.
Bullet to the head didn't work.
The hosts of zombies that have been created over the past couple of decades are unlike anything ever seen before. They are fast, some lightening fast. They are brutally strong, and large in size. They are also extremely intelligent and once transformed, capable of utilizing machinery and other technology.
There have been discussions with regard to a new-aged zombie group that has been utilizing the Internet. They have created social networking pages where they have been able to recruit potential new members. They are able to track and find those that they need or want to add to their clan. This is somewhat frightening as there is no real policing or tracking of this. There are further warnings and concern that they are planning for the ultimate zombie attack.
As you can see, the zombies of our time are much different than those that were created and wandered the earth centuries ago. In the past one person could likely manage to take care of 8-10 zombies by themselves. It was not hard to elude them, confuse them, and ultimately eliminate them. Today however it is becoming harder and harder to deal with the changing evolution of the zombie.
It is recommended that everyone study up on this. Depending on the nature of the attack, preparedness is your best bet. There are a number of resources online and in government and university libraries that will assist you in creating a plan of action. It is essential to know what you are dealing with in order to protect yourself and your loved ones.