If you mean you work for the company, use "a job with the company". You could also say " in the company", but that's usually used to describe the department within a company for which you work. I have a job with Nike. I work in marketing. "A job of the company" has a different meaning. It refers to a job performed by the company itself, not by an employee, but this would almost always use "the job" rather than "a job". The job of the company is to produce shoes for Nike.