Ok so i have installed World of warcraft (litch king) and i am ready to play it after 5 months that i had quited

I got one problem that "scares" me thow

i am about to buy a membership card...but before i do that i have launched WoW to see if it is woking....

it works fine... but... on the thing that u sighn in is says enter your battle.net account???

i never had a battle.net account....

do i need to go and make one or do i need to enter the information that i had created on the original world of warcraft site?

again i am going to play this game after 5months that i had quited....but i am afraid to go buy a membership card cause i do not know were to go and upgrade it? on the original WoW website? or i need to make an account on battle.net?

please reply!