Setting:
Probably in an urban or sub-urban area.
This would create plenty of zombies to be afraid of, as well as plenty of places to scavage for food and such, or to use as hiding places.

Rural areas could work, but the amount of zombies would be fairly low, most likely, due to the lack of people in general.

Wilderness. Pretty much out of the question, unless your zombie horde is primarily composed of animals. In which case the horde's size is still limited, probably more so than in rural areas (farms can have plenty of animals).

Climate and such is mostly a secondary concern, although it can, and probably will, be a major factor once the type of zombie is established.

The Zombies:
Can't have a zombie apocalypse survival without zombies, one would assume.
So erm, zombies. The type of zombie can go all over the scale, from rather realistic (as realistic as zombies can get, of course), meaning that they'll most likely last for only a month or two at most, due to natural decomposition, or other causes. In which case it would most likely be a case of surviving long enough for time to take its toll on the zombies, and wipe out what remains. Possibly creating a cure or some such.

On the other end of the scale, we got zombies which decompose, but aren't botherd by it for some quaint reason, as well as getting up again after a couple of hours/days, unless truly destroyed. In which case you'll need to make sure that no new zombies are created, as well as making sure that every last zombie gets destroyed. Lot of effort, and the PCs are mostly doomed from the start.

Then there's the matter of zombie intellect. They're frequently shown as being too dumb to use anything (doors, ladders, etc.), while on the other hand can be shown to use weapons at the same time (pipes, clubs, knives).
In rare cases they're even known to use firearms (and picking them up and switch magazines).

Cause:
Eh, can be as varied as your imagination allows.
Realistically, you'd be looking at either a mutated virus, or alien influence in case of modern RPs. Or dark magic and the like as possible causes in fantasy RPs. What if magic exists in an alternate modern world?

Characters:
Preferably, rather average people of which a city is normally composed of.
So no ex-navy seals and the like, unless they got a pretty obvious disability which caused them to quit the navy seals in the first place. No zombie-killing machines, in short.