How can I add my card. To use my card on the app??

This document explains how to add a payment method to your Apple ID on a Mac, Apple mobile devices, PC, or online. Click here --> Add a payment method to your Apple ID - Apple Support

On an iPhone:

Open the Settings app.

Tap your name.

Tap Media & Purchases, then tap View Account. You might be asked to sign in with your Apple ID.

Tap Manage Payments.

Set up Apple Pay - Apple Support