Ok this is what I would do:
At first you might want to collect Fadeleaf. This herb requires a skill of 160 so you can easily get it up to like 185. You can find some Fadeleaf in Swamp of Sorrows.
At a skill of 185 you can gather the herb Khadgar's Whisker.
You can find some of it in The Hinterlands, Arthai Highlands, or you can also stay in the Swamp of Sorrows, as there is a decent amount of Khadgar's Whiskers there. You can use this herb to get your skill to 205.
At a skill of 205 you can gather the herb Firebloom.
You can find it in Searing Gorge, Blasted Lands, Tanaris,...
You can use this herb to get your skill to 230.
At a skill of 230 you can gather Sungrass, which is found in places like Feralas, Ferlwood, Azshara, The Hinterlands,...
At a skill of 250 you can gather Gromsblood, found in Felwood, Desolace, Blasted lands,..
At a skill of 270 you can gather Dreamfoil, which can be found in places like Azshara, Un'goro Crater,...
At skill of 285 you can gather Plaguebloom, found in Eastern and Western Plaguelands, and also in Felwood.
This is just one way to get to 300 herbalism. There are tons of others.
You can easily find the skill requirements of the herbs and the places to find them by using this chart and/or this map.
http://www.worldofwar.net/professions/herbalism/herblist.php
http://mapwow.com/
Good luck =)