I would recomend buying 2 world of warcraft battle chest.
Basicly you need two sperate accounts.
Account A
Account B
One for you and your girlfriend.
Each account needs to have a WoW key activated in the account management.
Now the battle chest has the original game key + Burning crusade expansion. Which will let you get your characters to level 70.
Log into your world of warcraft account management, and click upgrade wow and type in the key that is included in the battle chests.
Do this for both your accounts.
Once you get 70 you can buy the next expansion wrath of the lich king.
Battle chest looks like this.
http://i734.photobucket.com/albums/w...5/wowchest.jpg
Hope this helps.