Once you have chosen the product you want to buy, put it in your shopping cart. Then go to cart and proceed to completing the purchase.
You will be requested all the information to register as a new user and billing information.
There is also a button to access if you are already a registered user. In this case, you are however required information for billing purposes. You must enter this data, or if you've previously entered them, confirm.
You can make the payment via PayPal.
You don't need to be registered with PayPal, all you need is a credit card enabled for internet payment: in PayPal payment page you have two options: pay with your PayPal account or, if you don't have a PayPal account, directly with your credit card, clicking on Don't have a PayPal account? See also this article.
Completed the order you will receive an e-mail with the order details.