Instructions
Power on the Nintendo Wii and press the "A" button when the black warning screen appears. This will take you to the main Wii homepage.
Point your controller at the small circle in the bottom left of the screen that says "Wii." Press "A."
Select the large box on the right side of the screen that says "Wii Settings." A menu will appear with a column of white buttons.
Locate the small circle halfway down the screen on the right side of the menu options and press it twice. This will change the options that appear on the screen. You can also press the "+" button on the controller rather than click the circle. You should now see a column of menu options with "Format System Memory" at the bottom.
Click the "Format System Memory" button and select "OK." The system will warn you that all of the data saved on the Wii System will be erased.
Wait a few minutes for the system to complete the format. The time it takes to complete the process will vary depending on how much data is saved on the system. Once it is complete, the system will restart and run as if it had just come from the factory.