Speak with Warlord Gorchuk in Orgrimmar's Valley of Spirits.
Hello, I'm glad that you've decided to hear me out. The Horde needs all of the help that it can get to prepare for the Ahn'Qiraj War, and that means that we need you! Even now as we speak, official collectors are gathering the necessary material needed for the upcoming war, but we won't be able to meet our goals without your assistance, [player]!
You should go speak with the guy in charge, Warlord Gorchuk. What do you say, [class]? Will you help out with the vital preparations?