How to subscribe to world of warcraft?

well you see when we bought the game i payed but my dad accidental only paid for one month and i want one month recurring but really i have no idea how to. i go to my account on battle.net then i press settings and then go to my payment options but where is the option to choose for how long you pay and my mum wants to see how much money we pay.. so is there another way or should we just do it this way?