The sticky at the top : Herbs, where to find them, etc etc, has all the information you'll need and then some.
When you reach lvl 50 and 300 herbalism, get a port to Shattrah City, take your mount over to Honor Hold and pay 10g for the lvl 375, go back to the old world, and you can farm high lvl items like Golden Sansam past 300 to skill you up in the old world till you get to Outlands and can get Felweed, Dreaming Glory etc etc to get you to 375.
Its very easy, I took my lvl 60 from 0 to 300 herbalism in about, oh 3 hours.