When you cannot find a full-time job, mini-jobs of less than 15 hours a week are a good way of earning a little money and getting to know new people.
The same rules apply to employment by the hour as to a proper job. You also need a work permit for this, which you can apply for from the immigration authority ("Ausländerbehörde").
Get to know German companies
Mini-jobs, like internships, are a good way of getting to know a company and getting an initial impression of working life in Germany. Your German skills will improve by themselves at the same time, because you get constant practice through your contacts with German colleagues. If you work as a domestic help, look after small children or help out in the garden, your employer must register you. After that you can apply for a work permit.