You can create a Nintendo Account for free using a smart phone 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 e-mail address is required to create a Nintendo Account.
・ We recommend you use an e-mail address you intend to continue using in the future.
・ Please keep in mind that if you lose access to your e-mail address due to switching phones or some other event, you won't be able to receive 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 User info menu. If the Account Management 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.