You can say I work in a garage or I work at Joe's Garage [ name of the company]

Agree with Jura that a mechanic would say either. One more thing... The place where a mechanic works to fix cars is called a "garage". The structure attached to my house where I park my car at night is also called "garage". If I was working in my garage at home, I would not say, "I work at my garage" (awkward). I would say, "I work in my garage."