What is often the setting for a dystopia?

With in a dystopian story the protagonist generally tries to go against the leader of the society. Sometimes He/She is helped by a group of rebels who are also trying to go against the leader of the society. 
Other times He/She is just trying to escape from the Society since he no longer has a reason to stay.
When writing a Dystopian Novel the story can start a few different ways. 
One way is to have the protagonist start out believing that the society is a Utopia and through their travels or experiences discover that the society is not what it seems to be.
The next is to have the Protagonist in the story start off already with a sense that something is going wrong. 
Since Dystopian novel often take place in made up places it is often necessary to give a back story to show how that world was able to develop from our world to become how it is.
In dystopian novels the story is often left unresolved. for example the protagonist will try to get his/Her point across but often fails or He/She ends up escaping but you are left with a sense of wonder since you never know what happens to the society that He/She escaped from.
The general idea of dystopian book is when
a person or a group of people gain enough power
that they begin to control society. Generally the protagonist is someone who feels that their is something wrong with the society and sets out to change it.

Hope this helped :)