Kriss, once again- I will not claim to be an expert in this area, but I know a little. To be a "mechanic" you just need to hang out a sign on the door of your shop- no other training or certification is necessary. There are a couple of "certification" organizations for mechanics (most seem to prefer "technician" over "mechanic"....), the most prominent seems to be ASE (National Insititute for Automotive Service Excellence), the rest are mostly Marque-specific, like BMW trained, or GM trained, etc.
To be ASE certified, you have to have a certain level of hands on experience, I think it only 1 year, and you have to take a series of 8 tests in different specialties. These include- brakes, suspension and steering, Auto transmission, manual transmission, engine repair, engine performance, heating and AC, electrical and electronic systems. Once you pass ANY ONE of these tests, you are "ASE Certified" and you can wear the patch they send you and display the ASE signs all over the shop, once you pass all 8 of these tests, you are certified as an ASE "Master Automobile Technician".
Now, the tests are NOT that hard -I decided to take them a few years ago, with little actual hands-on experience in a shop setting, I took 6 of the 8 tests in one sitting, and passed 5 of them- missed the 6th by 1 incorrect question. I then took the remaining 3 at the next testing date, and passed them all, so I am now a Master Auto Tech!!........FWIW. IMHO, if I went to a shop that did not have a bunch of MASTER techs on their staff, I would keep looking. If there were only 1 or 2 guys who had a couple of certifications, I would keep looking. Any tech that cannot pass all 8 of these tests would not work on my car. I hear alot of techs saying that these tests are a joke, and they don't waste their time with them- which may or may not be true, but if they don't have some sort of proof of their qualifications, I would not choose them.
Now, there are also Technical auto repair classes + courses at local community colleges, as well as some private specialty "automotive" colleges scattetred throughout the country- I cannot comment on those, as I have no experience with them.
HTH, Bret.