You have to buy Both WOW and Burning Crusade.
I advise you to look online on Ebay or Yahoo SG Auctions for second
hand accounts, there are some reasonably cheap ones with almost the
same price as the originals.
If you have credit-debit card, on installation, they will ask you
to register on the Blizzard website and enter ur card info. After
that, u select ur subscription either for 1 month, 6 months, etc,
then you can instantly play.
IF NOT, when u buy ur game, buy also a GAME CARD, with a unique
code at the back. Go to e website and register as per normal, then
enter in ur card no for a 30 day subscription.
There are many ppl quitting WoW and BC nowadays, I think u can get
cheap accnts second hand (: Hope this helps