However, there is one exception: Working as a student assistant or academic assistant ("Hiwi") at university is always allowed, provided that you do not work for more than 20 hours a week, as specified in the Lower Saxony University Regulations (§ 33 NHG).
Student assistants work e. g. in libraries, teach tutorials, or assist professors and research projects in their (research) tasks. These jobs are usually very compatible with your studies and can even benefit your course work – but as a consequence, they are also very popular among students.
For more detailed information on student jobs, visit the German National Association for Student Affairs ("Deutsches Studentenwerk") or the DAAD, or ask your university's International Office. They can also give you tips on how to get a job.
FYI: There are also special legal regulations for (mandatory) internships as part of your studies! For general information regarding internships, please visit the DAAD website, or contact the German Employment Agency for details.