It depends on the shop where you want to work. I started working at a local oil change place when I was younger and then went to a shop with very little knowledge or experience. Over the last 8 years I have taught myself how to do everything from replacing axles to engines and transmissions. The main thing is you should be a mechanically inclined person. If you don't like to take things apart and fix things then it's not the work for you. Some shops require experience while others will hire someone with little to no knowledge and help train them.
You also have to be prepared to spend a fortune on tools. As a mechanic you have to have your own tools. Over the years I have spent well over $25,000 on tools and toolboxes and there are a lot more tools that I need to buy. It is a never-ending process. You can never have all of the tools that you need. The car companies are always coming out with new ways of making things and more tools are made to keep up with this. It is a huge investment of your time and money. It's a decision not to be taken lightly. Just to get the basic tools needed to start you will be looking at over $500.
As far as certifications, A.S.E. (National Institute for Automotive Service Excellence) is the industry standard for testing and certifications. There are many tests that you can take for different aspects of auto repair/maintenance. To become A.S.E. certified you have to have at least 2 years of verifiable experience in the auto repair industry. The registration fee is $36 and each regular test (8 tests) are $30 each. Advanced level test (2 tests) are $60 each.