It's done through subscriptions so you need to have a credit card. You simply enter your cc number into the website- select which option you would like (1 month recurring, 3/6/9 months etc) and EA will apply that to your account. As far as I know you can't set it up over the phone but there may be time cards (where you head to a walmart or something and buy a physical code which you enter.)
Well when you buy it first. It's a key to activate the game. Once you have the game, when you want to play every month, you gotta play the gamecard or put a auto-subscription using your PayPal to the subscription on SWTOR website.