To start gaining reputation with one of these two factions, you're going to be doing some heavy duty questing in Sholazar Basin. You'll want to be at least level 76, because that's when you can get the quest Where in the World is Hemet Nesingwary? Yep! Old man Hemet, him again. You'll get this quest in Dalaran from Archmage Pentarus, who will send you to Hemet's camp (sort of), and that's where you're going to be questing for awhile. Talk to the crew, do what they ask you to do, eventually you'll be sent to a pretty little spot called River's Heart.
At River's Heart you'll find a Human chickadee by the name of Tamara Wobblesprocket. Yes, Wobblesprocket. I think she hooked up with the Gnome standing a few feet away from her. Go ahead and vomit for a few minutes before you go on. Done? Excellent. Okay, Tamara Wobblesprocket offers the quest The Part-time Hunter. Go find your prey, and when you do you'll find a Frenzyheart nearby that offers you the opportunity of a lifetime: Be a slave to some badger-dudes!
If you're looking to be an Oracle in the end, do not worry. Do this quest, and the entire quest chain. You'll be gaining Frenzyheart reputation and losing Oracle reputation, that's okay. Don't panic. You know how the Aldor/Scryer situation felt sort of... random? You didn't really know the full extent of their backstory and intentions until after you made your choice? You sort of flipped a coin to make your decision unless there was a rep reward you found on Wowhead you really liked? Yeah, they fixed that here.
This questline is very long and introduces you to both the Frenzyheart Tribe and the Oracles. You learn a lot about each of them, and you get an opportunity to see what their Quartermasters have to offer. At the very end of the quest chain is when you're finally faced with your choice. It's actually a very, very cool ending that I don't want to spoil, so I"ll just leave you with this: If you screw up your choice in the heat of the moment, don't turn in the quest. Just do it over again and choose the other side.
Q: So... which side do I pick?
A: If you're a big jerk, go Frenzyheart. Jerk.
Q: No really, who do I pick?
A: If you want to decide based on the items they offer, just hit Wowhead as usual. If you want to decide based on their stories and backgrounds, go Oracles unless you're a huge, massive jerk. Then go Frenzyheart.
Arthas awaits and so do your questions. Find the answers you've been looking for that will help you with your journey into Northrend and to level 80 with Wrath 101.