You walk up to the building in the middle of the city and open the door. Just ahead is the Pokemon Center, where you can adjust you team, use items, view team stats, and of course, heal your pokemon.
At the end of the hall is the Poke Mart,where you can buy items. A sign is on the door reading:
To your right is the Gym. This is where you can challenge Trenton, the Gym Leader. Across from the entrance to the Gym is the Hall of Fame, where you can see a list of those who have beaten Trenton.
To the left is the Transport System that will take you to Maelstrom City, Home of the R.E. League!
You may also turn around and leave City Hall to go elsewhere in the city.