While there are no "official" qualifications for being a blacksmith, you're not likely to attract customers if you don't know how to make anything.
To gain exposure to the craft, you can take some classes at local art schools, community colleges, and historic centers. Typically, classes are offered at beginner, intermediate, and advanced levels. Once you have some projects under your belt, you can take further classes in making specific decorative items. Unlike, say, chemists or engineers, you really get to customize your education down to the stuff you actually want to learn.
After taking some classes, blacksmiths may apprentice under a professional artist. It may take a couple of years to pick up additional blacksmithing skills, not to mention an understanding of the business side of things. Just make sure you save money on the side during these years, because opening up your own studio isn't cheap. And iron is the only thing you want to get in the habit of forging.