Use Your Gamer Launch Account
Your account from any Gamer Launch site is valid here.
If you already have a Gamer Launch account connect your Facebook account on your user profile first before using Facebook connect to login. Otherwise you may end up with a new account.
There will be lots of ways to level and get geared up in MOP. Besides just quest grinding, you will also be able to run scenarios, challenge mods, world bosses, dungeons and raids. To read up on what is coming visithttp://us.battle.net/wow/en/game/mists-of-pandaria/feature/#levelcap