When I was a mechanic, we had many technicians where I worked who simply started working on cars when they were young, and then got hired by a garage. The garage would typically allow someone to work on what they seemed to be able to do. And more senior mechanics would mentor them, of sorts. They would gradually be given more responsibility for more complicated and demanding jobs. Some were initially self-taught and some went to trade schools. As time went on, larger garages and especially automotive makers would develop their own training programs. A Certified Mechanic was a very good thing to be. And cars -- and other vehicles -- are complicated enough to demand a lot of different talents and areas of knowledge. Especially today when technology used in automobiles is so advanced. So if you're starting out, I would recommend getting some experience from any of those sources -- school, trade schools, personal experience, or doing more menial things in a garage somewhere. At a certain point, you'll be encouraged to get certification from some source or another, and the garage will certainly point you in the right direction. The bottom line answer here is, "experience", and you control how you get that. Besides, if you really like being a mechanic, you'll already have found yourself getting that.