Most students can do with a bit of extra income. But this should never involve more than some casual or temporary work, otherwise the studies might suffer.
Most international students are only allowed to work max. 90 days per year, alternative 180 days part-time up to 4 hours per day without having to obtain a work permit.

University jobs

Jobs in the departments, libraries, or maybe with the student services office or with other offices and facilities at the university tend to be snapped up quickly. Please remember that you will need a work permit from the local employment office (Arbeitsamt) if your work-permit-free employment exceeds 90 days (alternative 180 days part-time up to 4 hours per day).

The work permit regulations relating to international students are very strict indeed. You may be expelled from the country if you contravene these regulations.

Further details and information on these regulations can be found in
”DAAD Information on the Statutory Frameworks applicable to the Pursuit of Gainful Employment by Foreign Students, Academics and Researchers"
(http://www.daad.de/deutschland/download/03118.en.html)

Please also remember that casual jobs are not so easy to find, because the unemployment rate is high in Germany.