Add a card for payments

For what type of payments?

For App Store and iTunes payments follow the steps in the support document linked below.

click here ➜ Add a payment method to your Apple ID - Apple Support

For payments through Apple Pay click here ➜ Set up Apple Pay - Apple Support

For Payments towards your Apple Card balance click here ➜ How to make Apple Card payments - Apple Support

If it's something else, please explain.