World of Warcraft: This and That

Leveling from 150-170

Head to the Wetlands or Hillsbrad Foothills (although this guide will have you head to Arathi Highlands next) and level to 170 by collecting Briarthorn, Kingsblood and Liferoot. After Wetlands you'll be right next to Arathi Highlands where you'll stay until 225. You can start selling these herbs on the Auction House.

Wetlands Herbalism

Hillsbrad Foothills Herbalism

Leveling from 170-225

Arathi Highlands is an awesome place for leveling herbalism once you get to 170. You'll spend a lot of time there gathering Wild Steelbloom, Bruiseweed, Fadeleaf, Goldthorn, Liferoot and Khadgar's Whiskers. Stay there until you are level 225, and don't leave earlier because you'll have a rough time finding a more efficient place to level.

 
Arathi Highlands Herbalism
Once you're 225/225 you can learn Herbalism up to 300 in Booty Bay. Both factions can to visit Flora Silverwind, who can be found below and to the east of the Alliance Gryphon Master.

Leveling from 225-275

Head to Feralas and start gathering Goldthorn and Liferoot until you're level 230, then you'll be able to start collecting all the Sungrass you keep running by. Collect Goldthorn, Liferoot, Sungrass, Purple Lotus, and Golden Samsan, for this guide stay until you're 275. Alchemists need a lot of Sungrass while leveling to 300, so make sure you sell it for a lot of gold on the Auction House, and check out this guide for more on making gold with Herbalism.

Feralas Herbalism