Most dystopian fiction is based around a contemporary political or social situation and functions as a reductio ad absurdum thereof. In other words, you want to look at a current trend you dislike and imagine what would happen if it were taken to its logical extreme as a way of showing the problems with that trend. For example, you might start with the premise that social media like Facebook are increasingly intrusive; it's increasingly difficult to maintain online privacy. What would a world be like with absolutely no moments of privacy? Once you have imagines the dystopian situation, you create a protagonist who comes into conflict with the dystopian system you have invented and select either a happy ending (triumph over the system) or sad ending (protagonist conforms or dies).