I will explain nao!

Wii Speak has a friend code which your friend has to add. You both have to add each other's friendcode.

If you start a chat on the Wii Menu and then join a non-Wii Speak compatible game you will still be able to chat, As long as you start the chat in the Wii Menu it's fine.

If you want to chat with others online in a game like Animal Crossing: City Folk then just join a game and turn on your Wii Speak while online.