To play the full game you need to get all of them.
But if you wanna get straight into playing until I think level 80
Go and get the Battle Chest
It includes;
The Base Game - 60
The Burning Crusade - 70
Wrath Of The Lich King - 80

You can pick it up of Amazon for around 15 GBP or Digital copy for around the same of battle.net

Once you max out with them you will need
Cataclysm -85
Mists of Pandaria - 90
And soon you will need Warlords of Draenor - 100

The - ## Is what level you can play up until.

Hope this helped.

The Battlechest is probably your best bet for starting off, it usually includes a months subscription and a couple of the older expansions which you'll need anyway.

you are best going through them like this

Burning crusade > Wrath of the lich king > Cataclysm > Mists of Pandaria

Thanks for your quick responses. Really helped me . One more question though, can I carry my trial character through or will I need to make a new one?

iBraaad wrote Thanks for your quick responses. Really helped me . One more question though, can I carry my trial character through or will I need to make a new one?

You'd continue on whichever characters you already have if you want to, provided you continue to play on the same server your trial chars were on.

Okay, great. Thanks for your help everyone

if you get the battlechest you get a free month and you get the following expansion packs: Burning crusade, Wrath of the lich king, and Cataclysm. so you will be able to play up to level 85, then if you want to play to level 90 you will have to buy mists of pandaria

From what I know, the base game, TBC, and WotLK, are all on the WotLK install disc.

I might have an extra code to get you free expansions (up to cata I think) and a free month. Message me.