What is a Nintendo Account?

A Nintendo Account is required for using Nintendo services such as My Nintendo. 
Please visit the below page to create an account for free.

• Nintendo Account: https://accounts.nintendo.com

To create a Nintendo Account through the app, please follow these steps:

1. From Menu on the title screen or the Misc. menu within the game, select Account Management.
2. Select Create/Link a Nintendo Account.

Note: A Nintendo Account is distinct from a Nintendo Network ID.

Related question:
【How can I link Fire Emblem Heroes to a Nintendo Account?】