Nintendo has been making strides in collecting all of its disparate console and handheld user accounts under one umbrella, and – in theory – the account you complete for the Switch should be the last Nintendo account you have to make.
If you set up an account with your Wii U, 3DS, or for any of Nintendo's mobile games, then you're halfway there. The last step, for the Switch, is to set your User ID. You can do that by going to your Nintendo account, logging in, scrolling down to User ID and hitting the affiliated edit button. From there you can select your User ID screen name. It's a good idea to go ahead and grab the name you want before someone else does.
The Nintendo Switch launches on March 3, and right now we're still unclear how the console's online component will work.