Well, City Championships in the United States are held between November and January.
In order to run a City Championship, you have to show that you have experience running tournaments. Are you a tournament organizer? If not apply through your "My Pokemon" account at go-pokemon.com. Once you have been approved, run several tournaments. Then in September or so, send an email, probabily to customer service through the go-pokemon website and request to run a City Championship.
Once you are approved as a tournament organizer, a new area of your "My Pokemon" account appears where you can sanction tournaments.