Answer:
Complete daily quests for the faction you are aiming to gain reputation with, on the quest hand-in you will be notified in your chat log of any reputation gain (Assuming you haven't changed the default settings).
The second method of gaining reputation with a faction, is to complete their quest line in the respective area up until friendly and track down their quartermaster, he will sell you a tabard which you can equip, this will allow you to gain reputation with faction for killing enemies in any level 85 dungeon on normal or heroic.