Once you have created a account go to the MY SUBSCRIPTION page, scroll down to the Manage subscription section,

1. Enter credit card information, by clicking the red button "Create or Modify Payment Method" , this card will be used to charge your subscription every month until you cancel.  

2. Once you have entered the credit card information then click the green button "Select your subscription" , that will open up a popup where you select the plan you want, and then click blue button "Update Subscription".

3. If you want to cancel your paid subscription but keep your account open on the free level then just redo step 2, Click the green button "select your subscription" and pick the Free level, and click update.