Quick Breakdown:
1 - 50 Peacebloom, Silverleaf, Earthroot
50 - 100 Mageroyal, Briarthorn, Stranglekelp
100 - 170 Bruiseweed, Wild Steelbloom, Kingsblood, Stranglekelp, Liferoot
170 - 210 Kingsblood, Liferoot, Fadeleaf, Khadgar's Whisker, Goldthorn
210 - 270 Purple Lotus, Sungrass
270 - 300 Sungrass, Gromsblood, Golden Sansam, Dreamfoil, Mountain Silversage, Plaguebloom
300 - 325 Felweed, Dreaming Glory
325 - 350 Ragveil, Flame Cap, Terrocone, Felweed, Dreaming Glory
350 - 400 Goldclover
400 - 435 Goldclover, Adder's Tongue
435 - 450 Lichbloom, Icethron
Don't forget you can pick up every herb you see, even if it's green.
First, visit any trainer in the main cities of old Azeroth - just ask a guard, and learn Apprentice Herbalism.
1 - 50 Peacebloom, Silverleaf, Earthroot[Peacebloom], [Silverleaf], [Earthroot]
Basically just every starter zone, they are filled with Herbs.
Durotar
Dun Morogh
Tirisfal Glades
Mulgore
You can also go: Eversong Woods, Elwynn Forest, Teldrassil And Azuremyst Isle
50 - 100 Mageroyal, Briarthorn, StranglekelpLearn Herbalism Journeyman.
[Mageroyal], [Briarthorn], [Stranglekelp]
Barrens
Yeah thats barrens, the Herb paradise. You won't find any other zone where there are so many herbs as in Barrens.
Silverpine Forest
You can also go: Redridge Mountains And Loch Modan
100 - 170 Bruiseweed, Wild Steelbloom, Kingsblood, Stranglekelp, LiferootWhen you reach 150, go to your trainer and learn Herbalism Expert.
[Bruiseweed], [Wild Steelbloom], [Kingsblood], [Stranglekelp], [Liferoot]
Stonetalon Mountains
Hillsbrad Foothills
Wetlands - the red zone is the best after you passed 150.
170 - 210 Kingsblood, Liferoot, Fadeleaf, Khadgar's Whisker, Goldthorn Learn Herbalism Artisan.
[Kingsblood], [Liferoot], [Fadeleaf], [Khadgar's Whisker], [Goldthorn]
Stranglethorn Vale
Arathi Higlands
210 - 270 Purple Lotus, Sungrass [Sungrass], [Purple Lotus]
The Hinterlands
After 245 you can pick up Ghost Mushrooms in the caves too.
270 - 300 Sungrass, Gromsblood, Golden Sansam, Dreamfoil, Mountain Silversage, Plaguebloom[Sungrass], [Gromsblood], [Golden Sansam], [Dreamfoil], [Mountain Silversage], [Plaguebloom]
Azsahara
Felwood
300 - 325 Felweed, Dreaming GloryVisit your trainer and learn Herbalism Master.
[Felweed], [Dreaming Glory]
Hellfire Penninsula
325 - 350 Ragveil, Flame Cap, Terrocone, Felweed, Dreaming Glory[Ragveil], [Flame Cap], [Terrocone], [Felweed], [Dreaming Glory]
Zangarmarsh
Terrokar Forest
350 - 400 GoldcloverVisit your trainer and learn Herbalism Grand Master.
Howling Fjord
400 - 435 Goldclover, Adder's TongueSholazar Basin
435 - 450 Lichbloom, IcethroneThe Storm Peaks