How to Redeem:
Go to Nintendo's website and make sure you're signed in to your Nintendo Account. There are options to sign in with your associated email address, your Nintendo Network ID, or via a social media account. After that, click here: https://ec.nintendo.com/redeem/#.
Enter your code then select ‘Next.’ You may be asked to reenter your account password. Select the 'Redeem' option.
You may also refer to this video link for reference.
How to check your balance:
Call: +1 (800)-255-3700
Abridged terms and conditions:
When you enter the card’s activation code on Nintendo eShop or other Nintendo shopping services, the card balance will be associated with your Account and will be non-transferable. There will be no remaining balance on the card.
Card balance may only be used on a single Account.
There is a maximum unused Account balance that may be stored on a single Account, and the card may, as a result, be temporarily unredeemable.
Card balance does not expire, and there are no fees associated with the card. This card is non-reloadable.
Balance only redeemable using a Nintendo Account or Nintendo Network ID (“Account”) on Nintendo eShop and other Nintendo shopping services to purchase games and other content.
Valid only with U.S. Accounts.
Chat & Email: https://en-americas-support.nintendo.com/app/contact