Once your card is registered with Apple Pay, how you use it will depend on which iPhone version you have:
With iPhone 6 or higher use Touch ID (1): place your finger on the sensor and move the device toward the reader.
With iPhone X use Face ID: double-click the side button, look at the screen and move the iPhone toward the reader.
(1) Apple Pay works with the iPhone 6 or above in stores, apps and on websites using Safari; with the Apple Watch in stores and apps (requires iPhone 6 or higher); with the iPad Pro, iPad (5th generation), iPad Air 2 and iPad mini 3 or above in apps and on websites using Safari; and with the Mac (2012 or more recent) on websites using Safari with an iPhone or an Apple Watch with Apple Pay enabled. To see the list of compatible Apple Pay devices, check this page.
On Apple Watch
Launch the Apple Watch app on your iPhone, select Wallet and Apple Pay.
Select Add credit or debit card and follow the instructions shown on the screen.
When you want to use your Apple Watch to pay, simply double-click the side button and move it toward the reader.
Once you register your card with Apple Pay, using this application to pay through the app itself or on websites is very easy: in the first case, you just have to select Apple Pay when it's time to pay and identify yourself with Face ID or Touch ID. If you're using Safari on your iPhone, iPad or Mac, just select it. You don't have to create an account or fill out endless forms.