how do i redeem a gift card for apple tv subscription

1. Add the gift card value to your Apple ID account as account balance (like store credit).
2. Pay subscription fees from the account balance. (This is sort of automatic: Account balance is used first, before other payment methods that may be linked to your account.)
3. Account balance is generic (not marked to one particular service), and can be used to spend on a variety of digital products and services.

How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support

Add money to your Apple Account balance - Apple Support

Check your Apple Account balance - Apple Support

What you can buy with your Apple Gift Card or Apple Account balance - Apple Support

Where is the gift card from? Look on the card, there may be a number to call on the back.