Many medicines were initially discovered by herbalists; these range from everyday things like aspirin (which was originally made by boiling the inner bark of willows), to important chemotherapy drugs such as vincristine.
Although their discovery may have begun with a plant, most modern prescription medicines don't contain any herbs; instead, they are made in the lab. This allows the drug to be more pure, and generally cheaper to produce. However there are many herbal remedies that can be bought at any pharmacy or nutrition store. Some contain whole plant material, while others may be a blend or a distillation such as a tincture.
There is indeed much training required to become an herbalist. Some herbs are extremely powerful, and should be administered only by those with training and experience. There are formal training programs for herbalism, as well as apprenticeships where you study under an experienced herbalist. Some states require herbalists to be licensed, some do not. For more information on becoming a professional, check out these links to the website of the American Association of Naturopathic Physicians and the American Herbalists Guild.