How do I add a card

It's a very quick process. Follow the instruction in this support document. I've copied the part of the steps in that document for iPhone:

Add a new card on your iPhone

1. In the Wallet app, tap the Add button .
2. Tap Debit or Credit Card.
3. Tap Continue.
4. Follow the steps on the screen to add a new card. If prompted, choose your bank or card issuer from the list or find them using the search bar.
5. Verify your information with your bank or card issuer. They may ask you to provide additional information or to download an app before approving your card for use with Apple Pay.
6. If you have a paired Apple Watch, you have the option to also add the card to your watch.