When Nintendo made the Wii, they added in backwards compatibility, meaning it doesn't just play Wii games, it plays games from an older system also, that being the GameCube.

Put the GameCube disc in. Believe it or not, that tiny GameCube disc is compatible with the Wii CD slot (Wii games use much larger disks). Putting it in is not as tricky as it may sound. Just insert it as you would a Wii game, with the cover of the disk (the part with writing, logos, etc.) facing right. It doesn't matter how far up/down you put the disk in, it will still be able to read it.

Go to the Game Channel on the Wii Menu and press start to begin playing. You cannot access the Wii Menu while in a GC game, so when you want to play something else, eject the disk and press "Reset".

Note:
Remember to open the two hatches on top or you may not see where to put the controllers and memory cards.
Take the disk out and reset when done playing