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 iPhone, open the Wallet app and tap the plus sign. From there, follow the onscreen instructions.
To add your Visa card to Apple Watch, open the Apple Watch app on iPhone. Next, select "Wallet & Apple Pay" and then tap "Add Credit or Debit Card." From there, follow the onscreen instructions.