Until you hit lvl 30, just make the best potions you can. To make a potion, just look at your herblore to see what ingredients you need, put first the correct herb, then the secondary ingredient into a water filled vial, and you'll have the potion.
Once you get to 30, continue making whatever potion you were making up to that point, because it would be a waste to use ranarr weeds to make defense potions.
As for the quests - unfortunately most of the quests that give herblore experience require a fairly good herblore lvl to begin with.
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Thanks Trainer Sapphire for the avy/sig.