While I'm hardly an expert on the subject, I've been studying herbs fairly intensely for about two years now. I guess my first suggestion would be to obtain a trustworthy book on the *medicinal* uses of herbs (which are very different from the magickal uses), since this will actually specify the effects of herbs upon the body when ingested. A good one is Varo Tyler's The Honest Herbal, which is a very comprehensive look at medicinal do's and don'ts. There are thousands of books on herbology out there, though, and any book that deals with medicinal uses will inform you about poisonous v. safe.
My favorite herb of the moment is Mugwort (artemisia vulgaris), which is a mild nervine, *excellent* for relaxation and clarity, without the sleepy effects of other nervines like valerian and skullcap. It makes *delicious* tea, especially if you mix it with licorice root and some kind of mint.
Anyway, blessings upon your discoveries!
Lover, poet, vagabond, warrier, witch