How do I add credit card to my Apple Wallet?

Open the Wallet app on your iPhone.

Tap on "+" button in the top corner

Select "Debit or Credit Card".

Then follow the prompts and you can either scan your card or enter the details manually. There will be a verification with your bank (via text email or a phone call). Once it's approved - usually within seconds - it is ready to use with Apple Pay.

See these instructions from the User Guide:

Set up Apple Pay in Wallet on iPhone - Apple Support

Have you already tried to do this and had a problem? If so, can you explain what happened? If you got any error messages, what exactly did they say?