Im thinking of fixing or selling cars for a living?
Once upon a time a guy with a box of tools he could carry in one hand could fix a car under the nearest shade tree.
In this day and age the cars have grown so complicated that it takes several thousands of dollars in equipment and a multi-year training course to be able to properly fix a car.
So, if you want to fix cars for a living you should go to a trade school. Ask the service managers of some of the local new car dealerships to recommend schools to you.
As far as selling cars is concerned new car sales are way down and a lot of dealers have gone, are going, and will be going out of business. On the other hand the used car business is good since people still need cars, but can't afford new ones.
I think you have to decide which career path you would like better; fixing cars or selling cars. Whichever one appeals to you the most is the one you should pursue. If you work at it either course can lead to a comfortable living.