Plants take from their environment--air, soil, and water/rain that nourish them. This becomes apart of them and many times is a big factor in the constituent makeup of their medicinal value. What this means is, if you want good German Chamomile, you want it to come from Germany. Rose hip oil from Spain will be superior in healing burns to rose hip oil from China. You can grow it elsewhere but it may not be as potent for the use it is so revered for. This is what makes herbalism tricky. For minor problems like cuts, scrapes, tummy aches, insomnia, etc. an herb is an herb as long as it is that herb. But for serious conditions you need to research well, find the herbs origin, and get it from that region.
As for learning herbalism, it's a long process. I suggest beginning with the German Commission E for the modern uses in practice today and use that as a basis. Then research each individual herb in science journals and studies made by universities through the web.