To start you need to get to at least level 5 and learn the herbalism profession. Already we know that herbalism is a very money ridden profession, and with that, comes the idea that with just a little effort, you can be rich with only a few hours work. This works the best if you are the Undead race, but also will work as Human, or Tauren. Like stated before, be sure to pick up the herbalism profession (and for added bonus, go with mining).
Now if you do go with herbalism, stick with theFind Herb skill most of the time, as the herb we are looking for sells more then copper at the lower levels. From this point we are now going to travel outside of the newbie area, and into the real world in search of, yes, you guessed it, silverleaf. Silverleaf is EXTREMELY abundant in the early and second areas of the game, but even more abundant in the Undead and Human areas.
Why this is I'm not sure, but after farming only one stack of silverleaf, you can sell it on the Auction House for as much as 1 gold 25 silver. Yes, for only at most 10 minutes work, 1 gold 25 silver for a level 10. Too good to be true? Well, the lowest I've seen silverleaf go for is 85 silver, so still even then, at that level anything over 20 silver is a lot, believe me, I know. You don't even need a map for this, as with the "Find Herbs" trait you can easily find those abundant silverleaf no matter where you are. All but some servers may have a more crippled economy in the lines of silverleaf, give it a few days and watch the price skyrocket.
Another great resource at higher levels are the ghost mushrooms. Your Herbalism level should be around level 245. Ghost Mushrooms are located in a little cave called Hinterlands. This guide can be used by both Alliance and Horde characters. You can easily gather about 10-25 gold per hour. Your character should be at least level 44, the higher the better however. The mobs in the cave are level 46-48.
Another great herb to find is the Black Lotus. The herb can sell for 5 to 25 gold depending on your server. You can easily get 30 gold per hour. Morning would be the best time to hunt for them since there's probably alot less players at that time. If you combine herbalism with mining you’ll be sure to make a ton of gold over a short period of time.