depends on horde or allaince as to where to start, but in general.
go to a starting area, pick peaceboom, silverleaf and earthroot until you are not getting skill ups. when you hit 75 skill, train for the next 75-150 levels
move to a 20's level area, pick briarthorn, mageroyal, and kingsbloom
move up to a 30's area, ....and so on and so on until you hit 300 skill.
then hit hellfire, train your 300-375 and get some skill ups in that zone.
your best place to start in outlands would be the dead mire in zangarmarsh. the bog lords there are herb-skinable and will yeild you skill ups faster then roaming around. a couple of plus's to the dead mire farming is...the bog lords give sporgore rep for a while and you get reputation while you farm. while killing the bog lords you will pick up lots of mote of life and greens and maybe a blue + all the junk you get for herb-skinn them.
now you are 375 herb skill and can go and pick any flower you want.