G. Mark’s Answer
I'd like to answer this with a slightly different approach. You need to love working on cars. That is essential. Once you have that, and I assume you do already, you'll do what Steve has said. Mechanics today are more highly educated than ever, largely because of the technological advancement of automobiles and machines in general. So certification is highly valued.
If you're looking into this sort of thing, trade school training will be enjoyable and easy for you. When I started out, I began work at a dealership and started just running cars back and forth to service bays and the lot or service advisor bays. As time went on, I was trusted with simple things like tire busting and undercoat rack. And then trim work and tuneups. A dealership for any large auto maker today will encourage you to learn more and more. There was a time when the mechanics would get a big case of service manuals for the new models, and we'd each get a book or two and read it. Nowadays, there is no possible way to contain all the stuff to describe all the technology in a modern vehicle in even a huge set of books. When I bought my first Cadillac Eldorado, I drove to the company that printed all our service manuals and got a set for that car. From the one or two volumes I had as a mechanic, this single car had twelve volumes. And of course, forget paper manuals today.
The point I'm making is that there's a lot to being a mechanic in even a few areas -- engine, chassis, electronics, trim, etc.. So you won't just someday "be" a mechanic, you'll be learning all the time on the job, because technology in cars is a real marriage made in heaven, and it's a blast.