How to add debit card in Apple phone

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

On an iPhone or iPad:

1. Open the Settings app.

2. Tap your name.

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

4. Tap Manage Payments.

5. Tap Add Payment Method.

6. Enter the payment method details, then tap Done.

You don't say for what purpose you wish to add the card, so: Set up Apple Pay - Apple Support