To add your card to Apple Pay, go to the "Finances" section at the main screen in the application, select the card that you want to add to Apple Pay, scroll down until you will see "Add to Apple Pay" button. You will be asked for your card details and accept the Terms and Conditions for the use of the Apple Pay service.
The other way, is to simply enter the wallet application on your iPhone, press the "+" button and follow the instructions. Apple will ask you for your card number and your card code - you'll find both in the application after clicking on the selected card in "Finances" section. Once you've put in your details, you need to approve our Terms and Conditions and verify your identity by entering the code we'll send you by SMS.