Start by clicking on your profile on top right of the dashboard and selecting Billing on the drop-down menu.
In the Billing menu, look for the Billing information section and click on the Add credit card button.
A window pops up in which you can fill in your credit or debit card information. Note, that in addition to the card and card holder information, you will need to select the country in which you operate and add the information needed.
When you're done, click Save card on the bottom and you're good to go!