To add your card details, just login to your account and click on Payment methods. From here, you can add your payment details and save them to your account. When you come to pay for your order, you can select one of your saved payment methods to pay for your order.

 

If you don't want to save your payment details to your account, then you can also enter them when you get to checkout. If there is no payment details saved to your account, then you'll be asked to enter them when you click Place Order.