Nothing. Just cook good nutritious food.

Bhaskar Shetty

I’m not sure on what you need to open one, but once you do, you might find this helpful in learning to market your catering business:

You can bake a basket of goodies each morning and deliver them to office buildings. Either you can have the receptionist announce over the PA system that the “bakeries” are here and have people come up front to buy. Or you can leave some at the front with a small locked box for payments (the honor system).
The city and county are not too worried about small operations like this. But if you get successful and hire 10 other people, you can talk to the city and county to make sure you are complying with the laws (no bug sprays stored near the food, butter and milk at 40 degrees temperature, no evidence of mice and cockroaches).

The key step is for your kitchen to pass the Department of Health inspection

Unfortunately, they often require that food-preparation business run from home must have commercial grade kitchen. Most homes do not have that, and cannot pass this requirement. So they rent a commercial kitchen space (e.g. church kitchen) to pass and get the DOH permit

Call your county clerk’s office and inquire the requirements you need to start this business

It is best to organize your business as a corporation or LLC because this is the type of business where you have a high likelihood to be sued . These legal structures separate your business from personal entity, thus the other party can only sue your business and not your personal assets

Think about insurance as well. This is very important