Answer:
When you first start a new game on City Folk, it will ask if you'd like to transfer your character from Wild World, or start fresh. You can select the option to transfer, and follow the steps it gives you. You will transfer your character over. Please be aware, any items in your pocket, your letters, or your accessories/clothing will NOT transfer with your character. The only thing that will is your character's name, hairstyle, and eyes.