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.
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.
・ 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.