to start wow from scratch
you need:
60day game card(for some reason its a must, we still don't understand why?)

the vanilla wow game code, which you can get from buying the game.
once you activate it, you get extra 30 day free game time,
in total, you get 90 days game time in your 1st start.

TBC game code,
note, you can get 2 of them(vanilla, tbc) in one buy in the world of warcraft: battle chest, its cheaper.

Wrath of the lich king game code.
then finally catalysm game code.

atlong as you have full patch, and active your account you can start play(but face a few limitation).


have the whole 4 installations is a good start,
you can get the vanila 10 day trial account 1st try it before you decide to buy the full set.
totally free, for 10 days.
but you can only level to level20, can't trade with other player,
can't create blood elf, dreanai, and death knight.