I've been looking everywhere for these damn quests and haven't found any :( Where do i start the quest line there? Thankyou in advance. :)
You should be able to get one from the quest board in Org. However, you need to be level 84 first.
If the quest isn't available for whatever reason, just speak with Garrosh (or Wrynn for Alliance), and you'll be sent on some errands, after which you'll be sent to TH. Once there, it'll be a few more quests before you're sent to collect mud and fight elementals in order to activate the portal.