How do I become a professional piercer?
First of all, you should know that being a piercer isn't as fun as it might seem. You often don't even make enough money to even meet the poverty line. You are at the whims of both laws and fashion trends—you could very well find yourself without a job or even a job market. If you spend the years you would have spent in college or at the start of a tradition "career path," as a piercer, you may well find yourself very much SOL when you hit 30. In addition, the work itself is extremely emotionally demanding, as well as putting you at constant and serious medical risk.
That said, there is one acceptable path to becoming a piercer that far outshines the other options. While it is true that a number of the top piercers are entirely self-trained, these are exceptional individuals that beat the odds. Realistically, you MUST seek an apprenticeship with a qualified piercer (if you apprentice under an idiot, you'll be an even bigger idiot than they are).
You can take a piercing course first, but understand that these courses are not training to be a piercer—they are preparatory courses intended as an introduction for those seeking an apprenticeship. In no way do they make you qualified to pierce the public. To get an apprenticeship, first find the best studios in your area (or best studios in areas you're willing to travel to). Get to know the piercers, and then let them know you're seeking an apprenticeship. Of course, they get a hundred kids a year asking this same question, so you may need to show perseverance and patience.