How do I add money on my Apple Pay

Apple Pay is a way of using a credit or debit card virtually from your phone. I think you mean Apple Cash. You can add to Apple Cash from a debit card that is already installed in the Wallet app. Tap on Apple Cash, then the 3 dots in a circle in the upper right and choose Add Cash. You can transfer cash from your debit card.

See this article to learn how to setup your AppleCash account:

Set up Apple Cash - Apple Support