WoW Account questions (please answer)

[Raw!]

Well I decided I would get WoW. But there are a lot of questions I'd like to ask.

So, currently I am playing on a trial account. And I already created an account in the website. Once my trial is up, I am pretty sure like most trials, something will pop up saying "Buy Warcraft now or Enter Code." So, once my trial ends, what should I do? I am planning to pay by prepaid cards. And my friend said I don't have to download WoW anymore, meaning I don't have to spend 40$$ on WoW, and just to spend on the prepaid cards. But the thing is, I'm not sure my trial version downloaded all of the stuff. So I really need help..