Animal Crossing: City Folk

You need to get a friend code from other people with the game. Then you can open your gate and visit towns. Go on the FAQS and ask for some friends or add any of your friends who have the game.

Mejo2be is partly right, because you also need your own friend code. To get it, you need to have a wifi connection on your Wii. from there, you talk to Copper, and ask him for a friend code. if you don't have one, he will look surprised and say you should have one registered immediately. He will connect to Nintendo WFC and issue you one. after that, if you go to your friend roster, you can register different people's friend codes. Then, you can invite guests, or go out to the other towns.

I've had this problem, I got my friend code from copper, and then to go to another town, the person in the other town has to had opened the gate, giveing you permission to go to their town

You get your friend code from copper.
Then, to go to another person's town, you and a person must add each other's friend codes.
The other person must open their gates though in order for you to go to their town.

you need to get friend codes of all your friends and put them in ur friend roster. MAKE SURE UR WII'S WIFI IS ON!