Pick a region that you have some knowledge of, one that is politically interesting for readers that you want to target. If you want to make the plot about a very familiar country like the U.S., you should try to find a regional base for the novel so you can determine who your characters are, what they do, where they come from and what the conflict of the story is.
Check out some of the major ongoing issues and conflicts in your selected region by simply reading a local newspaper or journal on a regular basis. Take note of any dialect that's used as well as references to major governmental agencies or organizations. Then take one of the major ongoing conflicts or political challenges in that region and start to do some research on it so you can become an ad-hoc expert on the topic.
Set the major conflict of the novel and then select characters who fall on either side. Your first and best developed characters should be your protagonists, or "good guys." On the other side of the conflict, develop the bad guys. A good guide to determining who is who is checking out what the majority of your potential readers think about either side of the conflict.
Tie in other political crises or government groups in order to spice up your political fiction novel with plot twists while adding an air of political savvy to the work. Use these opportunities to drive the action of the novel which, at least at some point, should be physical.
Tie up most loose ends but leave some dangling at the end of the novel to give your readers a lasting sense of realism since, in reality, political conflicts don't end, they just become less acute.