From the Undercity to Silverpine to Hillsbrad, then go east. Take the zeppelin to Grom'Gol Base Camp or take the Boat from Ratchet to Booty Bay. From theUndercity to Silverpine to Hillsbrad to Arathi to Wetlands to Loch Modan, then gosouth. From Stranglethorn Vale to Duskwood to Deadwind Pass, then go east.
First of all it should be mentioned that if you want to reach some point on the map while you're still low level, and the way is too dangerous, your best bet is to have a mage friend who can portal you to the capital cities for your faction. That should get you closer to where you want to go.
Horde Mages get the portals to Undercity and Orgrimmar at level 40, Stonard at 35, and Thunder Bluff at level 50.
You should be prepared to give the mage some coin. Each portal consumes a Rune of Portals, which costs 20s each. Of course this method costs money and you're unable to collect flypoints on the way. Still, if you simply want to reach Orgrimmar as a level 6 forsaken, just because all your friends are one of the three other races, and you want to level together, it's the fastest way.
If you want or need a travel route added to this guide, put your request in the discussion area.
The Horde homelands are divided between Kalimdor and the Eastern Kingdoms.
On Kalimdor, you'll find Mulgore, the Tauren homeland in the center of the continent, and the peninsula of Durotar, the Orc and Troll homeland, hanging off the eastern side. Thunder Bluff, the Tauren capital, is located in Mulgore, while Orgrimmar, the Orc and Troll capital, is located in northern Durotar. The Barrens are under near complete Horde control as well with the major town of Crossroads connecting the Orc and Tauren homelands.
On the eastern continent, the Undead capital of Undercity is located in the north of the continent in the Tirisfal Glades. In the Burning Crusade the Blood Elf capital of Silvermoon city is located on the eastern continent as well, at the farmost northern edge. In both cities there are Translocation devices that will quickly teleport you between cities. In addition, the Horde maintains an outpost in Stranglethorn Vale on the southern sub-continent called Grom'gol Outpost, which can be traveled to via zeppelin.
To travel between Mulgore and Durotar, you will pass across The Barrens, a large area with level 10-25 mobs. From Durotar, simply go west to Crossroads, south to Camp Taurajo, and west again. From Mulgore, just reverse these directions.
To travel from Kalimdor to the Eastern Kingdoms, or vice versa, you'll need to take a zeppelin. The zeppelins travel a triangular route between Orgrimmar, the Undercity, and Grom'gol Outpost in Stranglethorn Vale. The only other transportation between the two continents available to the Horde is the boat between Booty Bay and Ratchet.
There is now a Zeppelin between Orgrimmar and Thunder Bluff, and it should be noted that this zeppelin was not taken into account when this guide was written.
How To Get To Each Region: Horde
Silvermoon City / Eversong Woods
From the Undercity or Tirisfal Glades, go to the Ruins of Lordaeron. Head west from the central courtyard to find the Orb of Translocation.
From Stranglethorn Vale, go north to Duskwood, then go north again.
From the Undercity, go to Silverpine to Hillsbrad to Arathi to Wetlands to Loch Modan, then go west.
Tirisfal Glades / Undercity
From Orgrimmar, take the zeppelin to Undercity.
From Silvermoon City, use the Orb of Translocation to the Ruins of Lordaeron.
From the Undercity, go to Silverpine to Hillsbrad to Arathi to Wetlands, then go south.
From the Undercity, go to Tirisfal Glades, then south.
From Stranglethorn Vale, go to Duskwood, then go west.
From Elwynn Forest, go east.
From Stranglethorn Vale go north.
From the Undercity, go to Silverpine, then go east.
From the Undercity, go to Silverpine to Hillsbrad to Arathi, then go south.
From the Undercity, go to Silverpine to Hillsbrad, then go north.
From the Undercity to Silverpine to Hillsbrad, then go east.
Take the zeppelin to Grom'Gol Base Camp or take the Boat from Ratchet to Booty Bay.
From the Undercity to Silverpine to Hillsbrad to Arathi to Wetlands to Loch Modan, then go south.
Swamp of Sorrows
From Stranglethorn Vale to Duskwood to Deadwind Pass, then go east.
From the Undercity to Silverpine to Hillsbrad, then go northeast (behind Durnholde Keep).
From the Badlands, go west.
The Blasted Lands
From the Swamp of Sorrows, go south.
From the Searing Gorge, go south through Blackrock Mountain, around the perimeter of the molten span, to the south exit.
From the Undercity, go east.
From the Western Plaguelands, go east.
Durotar / Orgrimmar 1-10
From Mulgore, go through The Barrens and then northeast.
From Tirisfal, take the zeppelin to Orgrimmar.
Mulgore / Thunder Bluff 1-10
From Crossroads in the Barrens, go south to Camp Taurajo; follow the road west into Mulgore.
From the Barrens, go to north to Ashenvale, take the west road. Swim around Astranaar, and follow the road west, then north into Darkshore. The closest Horde flight path is in Zoram Strand, which is on the west coast of Ashenvale.
The Barrens / Crossroads / Ratchet 10-25
From Durotar, go west.
From Mulgore, go east.
Stonetalon Mountains 15-27
From the Crossroads in the Barrens, take the road west.
Ashenvale Forest 18-30
From the Crossroads in the Barrens, take the road north.
Thousand Needles 25-35
From the Barrens, go south.
From the Charred Vale in Stonetalon, go south.
Dustwallow Marsh 35-45
From Camp Taurajo in the Barrens, go south, then follow the road east into the marsh.
From the Great Lift in Thousand Needles, go west.
Tanaris Desert / Gadgetzan 40-50
From the Great Lift in Thousand Needles, go east to the Shimmering Flats, then take the road south.
From Ashenvale, take the road east.
From Raynewood Retreat in Ashenvale, take the road north.
Un'Goro Crater 48-55
From Gadgetzan in Tanaris head southwest through the desert. The Crater is west of Tanaris.
Winterspring / Everlook 55-60
In Felwood, become not hostile to the Timbermaw Furbolgs. Then travel through Timbermaw Hold in the north of Felwood. Take the East tunnel to Winterspring.
In Felwood, become not hostile to the Timbermaw Furbolgs. Then travel through Timbermaw Hold in the north of Felwood. Take the north tunnel to Moonglade.
Druids get Teleport: Moonglade, a spell which teleports the caster to the city of Nighthaven, Moonglade.
From Un'Goro Crater, head west.
From Stonard in the Swamp of Sorrows, head south to the Blasted Lands. Take the road south to the Dark Portal to Hellfire Peninsula (must be level 58 or higher). Take the flight to Thrallmar, then ride due south past Honor Hold. From there, go southwest to the Terokkar Forest. Head west to the city.
Alternatively, take the "Portal to the Blasted Lands" from any Major City, then continue through the Dark Portal as above.