In order to add other Wii consoles so you can send messages to each other and such, you'll need to add them to your Wii's address book. This can be accessed by clicking the mail icon on the bottom right corner of Wii channels screen.
Have your friends add you too.
If you want to play games, you'll need to buy the titles that have the blue Nintendo Wi-Fi logo on the upper left hand corner of the box.
Enjoy the forums.