Apocalyptic fiction is a sub-genre of science fiction that is concerned with the end of civilization either through nuclear war, plague, or some other general disaster. Post-apocalyptic fiction is set in a world or civilization after such a disaster. The time frame may be immediately after the catastrophe, focusing on the travails or psychology of survivors, or considerably later, often including the theme that the existence of pre-catastrophe civilization has been forgotten (or mythologized). –Wikipedia
You start with one Main Character who has either lost everyone and everything they care about, or who soon will lose everyone and everything they have ever cared about. This person needs a few skills that will make it logical for them to have survived while everyone else died. They are immune to the Virus, are an Ex-Seal or Special Opts, have a bunker with years worth of water and food-just in case, lives out in the middle of nowhere and don’t even know the world has ended-yet, or has great leadership skills that will allow them to Take Charge.
You then focus on Your Main Character and follow them around as they fight off evil hordes-search for food, water, and shelter-met the occasional Good Person-protect the handful of people that they have chosen to protect-and maybe try to rebuild the world.
As a general rule be vague about what caused the end of the world. A classic modern example is found in The Road:
“I knew this was going to happen.” Old Man
“You knew this was going to happen?” the Man
“Well, this or something like it it.” Old Man
How the world ends doesn’t really matter, just that the world as we know it ends. Since most of us are pretty fond of the world as we know it, putting an end to it is a very emotional experience. Even bad post apocalyptic stories usually get this part right. Our hero wanders around, starving and wearing rags. He walks into a grocery store, only to find it empty or filled with rotting food. He walks into a clothing store, only to find the clothes ruined by time and weather. He looks at empty farm land, only to see the fields have gone feral and he doesn’t know how to farm, or how to defend the farm from Them.
Our hero will almost always be on some kind of Quest. Whether this is an actual quest given to him by God as in The Book of Eli or a pointless waste of time Quest like The Man and The Boy heading for the coast in The Road. Give them something to do besides sit around and wait for the end.
There are two main choices here, go in search of something-food, other people, Sanctuary-or create your own stronghold and kill the evil things left roaming the world like The Last Man On Earth.
My favorite post apocalyptic stories end on a slightly upbeat note. Eli finds a small community of normal people who are printing books. BBC’s Survivors find colonies here and there as they wander around the UK. Both the Postman and The Road Warrior end with the fact that these are stories of the Bad Old Days and life is better now.
Of course, I also love a number of post apocalyptic stories that end with no hope at all. The Planet of The Apes ends with Bright Eyes discovering he was on Earth all along. Logan’s Run, the book, ends with out heroes heading out to space, since the computer running Earth has gone a bit mad over time. 12 Monkeys leaves us with a world in ruins as our hero can’t change the past after all. Various zombie movies leave the world to the zombies. And Cat’s Cradle left a handful of people alive on an island surrounded by ice.
What matters in a Post Apocalyptic Story is that our heroes face a new world unlike the old world. We share the end of their world with them. We mourn the loss of family, fast food, cars, warmth in winter, clean water, new clothes, and so on so forth. We face the prospect that our hero might have to kill someone to survive and we can ponder what all this means and what we would do in a similar circumstance.
One of my favorite scenes in Children of Man is when our hero meets with a Government agent who has moved into The Tate Modern and surrounded himself with works of art. My own idea was always to raid a military base and then move into the bunker that is the Kimbell Art Museum in Fort Worth. This would need to be a world where most everyone is dead and I can just walk in a pick up guns and ammo-but then why would need guns? To fight for the things that are still out there.
For a good list of post apocalyptic stories check out 60 Years of Post-Apocalyptic Fiction.