You can create a Nintendo Account for free using a smart device or a computer.
You can make one here: https://accounts.nintendo.com
Data charges may apply when you access the site.
You can also create one from in Fire Emblem Heroes by following the below steps.
■ Steps
You can register an account from the title screen by tapping Menu, or by tapping Misc. in-game, then tapping Account Management, then tapping Create/Link a Nintendo Account.
Notes:
・ An email address is required to create a Nintendo Account.
・ We recommend you use an email address you intend to continue using in the future.
・ Please keep in mind that if you lose access to your email address due to switching smart devices or some other event, you may miss important notifications.
・ If accessing the above link does not bring you to a new-account creation page there is a chance you already have created a Nintendo Account.
・Please check the information registered in the Account Management screen of the Misc. menu. If the screen says Manage Account Info instead of Create/Link a Nintendo Account, you already have a Nintendo Account and it is linked to your Fire Emblem Heroes data.