Companies have different rules about employees working other jobs. Study your employer’s rules regarding what they expect from you if you open a side business. If you intend to offer a competing product or service, you may risk losing your job, particularly if you signed a non-compete clause when you first started. You could even be subject to a lawsuit if you market a product or idea you came up with while working your full-time job.
I surprisingly get a lot of people asking to detail their car. I never intend to make a business of it, but I love doing it to my cars and people ask me to do it to theirs. All it takes is a cheap orbital buffer (mines a used craftsman) and a shop vac. I normally get easily $100 for a basic wash/wax/vac, or $200 to remove scratches and polish then wax the car.