Subscribing is very easy. You can do it either from Settings -> Billing or from the "Upgrade" button at the right-bottom corner in your builder. Once in the Billing page, all you need to do is select the plan you desire. The plans are very flexible, upgrade and downgrade is available at any time.
When you select a plan you will be redirected to the Billing and Card info page. Enter your CORRECT info and click the 'Subscribe' button.