Find a shop where the owner isn't a jerk and the employees are cool (which may be the hardest part). Start hanging out a bit, explain that you want to learn how to work on bikes and ask for an internship. Chances are you'll be the guy who fixes flats and takes out the garbage for a while but if the shop is cool and you're into learning things, you'll gain some experience and maybe even make a bit of money.
Sure, you can go to school and become certified - which may help you land a job at shops where it's a requirement - but I know more than a few techs who have completed school and like to talk a good game but are total idiots when it comes to actually working on bicycles.