Nintendo is now allowing users to create a new user ID for the Switch. Keep reading however, you might think that you already have an account if you have an NNID (Nintendo Network ID), however Nintendo been Nintendo you need to create a new user ID for the Switch.
You can sign into your Nintendo Account here, you are then able to create a username that you want to use for the Switch.
Simply scroll down to "user ID", and click edit. You'll get a verification code to your email address, enter the verification code and you can then create a 6+ character username.
For anyone using Outlook.com / Hotmail or an Office 365 powered email address you need to add accounts.nintendo.com and nintendo.com to your safe sender list otherwise the confirmation email will not even get to your inbox or junk folder.
So what are you waiting for? Get your favorite username now, before somebody else does!