You have to preorder. When you do, you can download the client and get access to the beta immediately. You also get early access to the game (which is currently set for Friday) otherwise you would have to wait until March.
On the top of the menu bar, where it says 'PREORDER NOW!' click that, it will navigate you to the pre-order menu.
Once on the menu, choose your Membership Type and from there the payment should automatically go through.
It's a clickandbuy basically!