To add a payment method, you need to be logged in and visit your account page:
The payment methods page can be found from here in the menu on the left hand side of the page:
After clicking on "Payment methods" you will be taken to a page containing all of your currently stored payment methods. Click the "Add Payment Method" button to proceed:
Adding a credit card will be the only available option here at the moment. PayPal is accepted at checkout and can be used for subscriptions at purchase but cannot be stored as a payment method here. Note that if you are adding a new payment method and have an active subscription, you can check the box at the bottom to change out the payment method on your subscription.
Once a new payment method is added, you can click the "Make Default" button to default to this option at checkout. You are also given the option again when making it the default to change your subscriptions to the new payment method if you have not done so already.