To make a Nintendo Network ID on Wii U, you must accept a User Agreement and provide information, including your birthday, gender, state, time zone, username, password and e-mail address. After that, a confirmation e-mail will be sent to your e-mail; confirm it to activate your Nintendo Network ID.
Your username can be between six and 16 characters. You can use numbers, letters, periods, dashes and underscores. You cannot, however, use punctuation as the first or last character of your Nintendo Network ID.