From the Wii home screen, click Mii Channel and then Start. This will take you to "Mii Plaza" where your Miis will wander aimlessly after you create them.
How to make a Diddy Mii
Click the New Mii button on the left side of your screen (it looks like a happy face with a "+" on it) to start a new Mii. You can also click the Edit Mii button (the happy face with the eye) to edit existing Miis you've made.
Choose the gender of your Mii. If you're lazy, you can click Choose a look-alike to open a screen of Miis to choose from, but it's more fun to click Start from scratch, which will open the main editing screen with a generic Mii to work on.
At the top of your screen is a row of buttons. Click the first one. This will let you fill in basic information about your Mii, such as name, date of birth, and favorite color (which, if you're making a Mii based on yourself, could of course be your name, date of birth, and favorite color).
