Apple Pay is the new feature that has been released with iOS 8.1. It lets you make payments securely with Touch ID. iPhone 6 and 6 Plus owners just need to show their devices at the NFC (Near Field Communication) supported terminals to pay.
You will have to setup Apple Pay by adding your credit card info to it. However, since you might already have a credit card on file (for iTunes purchases), you can link that to Apple Pay. This makes setting up Apple Pay faster and simpler.
How to Link Apple Pay to iTunes Credit Card
(Apple Pay is compatible with only iPad Air 2, iPad Mini 3, iPhone 6 and 6 Plus)
Step #4. Here you would be prompted to log in to your iCloud account.
Step #5. Tap on Use Card on File with iTunes.
Step #6. Enter the credit card 3-digit security code (CVV) in the Security Code field.
Step #7. Tap on Agree in the Terms and Conditions window.
That's all! You may have to wait for a few seconds before the card gets activated by Apple Pay. Once it is done and ready to be used, you will receive a notification.
At present, Apple Pay is available only in the US. According to media reports, it will be made available in other countries by next year.