If you want to require your customers payments to access your website, you may set the frequency of the payment under Options > Payment. Other settings such as the type, the cost, paypal email address, confirmation page, currency and discounts are also found under the same tab.
Payments may be required to access your website, or can be required only for certain user levels. User levels are set under the Levels tab. By default you have, VIP user, premium user and regular user as levels.