There are various opportunities for international students to finance their living costs.
International students are allowed to work for 120 full days or 240 half days within a calendar year. However, this work may neither impede on the purpose of your stay (studying) nor may extend the duration of your stay.
Only the actual working days are counted (including weekends). If the work does not exceed four or five hours per day, it is counted as half a day. A regular working day of 8 hours means 4 hours half day; a regular working day of 10 hours means 5 hours half day.
If you work more than 120 full days or 240 half days per calendar year, you should obtain the approval of the immigration authorities before.
Employment as student/scientific assistant
Student and scientific activities can be carried out without requiring approval and without time limits. This includes activities in the faculties of the university, at other scientific institutions, participation in research projects and also employment in organizations close to the university (such as tutors in dormitories of the Studierendenwerk).
A mandatory internship does not require approval if it is a required part of your studies (even if it is paid). This also includes the writing of final theses at a company. The 120 full/240 half working days that do not require approval are not affected and can be used additionally.
Voluntary internships are not a part of the curriculum and you will need to make sure that the 120 full working days have not yet been used up by other employment. If the 120 working days have already been used up, voluntary internships require a permit, even if they are unpaid. In that case, you must apply for the necessary permit with the immigration authorities.
Here you will find more useful information about working while studying: