Hiring the right employees is also important. Select "Employees" tab in Management screen. There, you can find a list, divided into three categories, people who seek work . Each person is described by two parameters: speed and quality of service , e.g. a cook has cooking quality value. To hire a person, you need to drag their name on the hours schedule and choose working hours for this employee.
In the beginning of the game you don't have much competition. This means that you should focus on employees who have a higher speed value rather than quality of service. Thanks to that you can increase your restaurant's income at a cost of your employees' payment. Also, they will be able to serve more clients.
However, you should stop doing that when competition appears in a city. Your clients can start going to other restaurants if they won't be satisfied with quality of service provided by your restaurant. The same goes for speed of service. Clients can leave your restaurant if they have to wait for too long. This also has an impact on your chain's ratings. Be sure that your staff has both values at, at least, 50%. Effectiveness of your staff and their cost depends on your previous income and how much you are willing to pay them.