If you have not set up a payment method on your account you will need to pay your invoices through the control panel.
After logging in your will see your outstanding amount in the Account box. To pay it simply click Pay Now.
If you would prefer to review your open invoices, click the amount outstanding instead.
Select the invoice you wish to pay for using the tickbox (if you have more than one open you can choose to pay just certain ones, leaving others open) and then the Pay Documents button. Follow the instructions to enter your card details and complete your payment.