If your Visa card is already connected to your iTunes account, just enter the security code when you enrol in Apple Pay.
To add a new Visa card to your iPhone, open the Wallet app and tap the plus sign. Use the camera to capture and enter the card information.
To add your Visa card to apple watch, open the Apple Watch app and iPhone and select 'Wallet & Apple Pay' and then tap 'Add Credit or Debit Card and follow the onscreen instruction.