In the secondary menu where it says "Hello {Your Name Here}" select Payment Methods from the dropdown menu.
Click on Add New Credit Card
Populate the details for your card. You can add an optional description for your own reference e.g "My Visa"
You will now be shown a list of Payment Methods currently securely stored (tokenised) in your account.
To set the default payment if you have multiple cards simply clicking Set as default will ensure that method is used for future payments unless otherwise selected for a service or item.