If you wish to work besides studying, you have the possibility to work part-time. There are certain rules you have to observe as an international student.
Principally, students from other EU member states, Iceland, Liechtenstein, Norway, and Switzerland can work under the same conditions as German students. If you want to work more than 20 hours per week, however, additional insurance fees must be paid.
Students from all other countries are allowed to pursue secondary employments under certain regulations only. We therefore recommend that you obtain information about the legal framework at an early stage, if necessary from the Foreigners' Registration Office (Ausländerbehörde). Generally, international students are permitted to work for 120 full or 240 half days per year and must not be self-employed or freelancers. If you want to work more than this given time span, the Employment Agency (Agentur für Arbeit) as well as the Foreigners' Registration Office need to give you permission to do so.
A research assistant can work without a time limit, which also needs to be confirmed by the Foreigners' Registration Office. You can get additional consultation at the Dortmund Student Services Provider and the International Office. The International Office also regularly provides workshops and career fairs to help you.
Bundesagentur für Arbeit
The Federal Employment Agency (Bundesagentur für Arbeit) is the official institution that is concerned with all employment-related matters. They have a comprehensive database of currently available vacancies to assist in the job search. You can also get personal consultation in one of their branches, one of which is here in Dortmund:
Agentur für Arbeit Dortmund
There is also a university team of the Arbeitsagentur that can assist you in your job search.
The list above just shows a selection of the many options available on the internet. There are multiple ways to find a job. Therefore, also check the local newspapers as well as bulletin and notice boards in universities, supermarkets and other public places for current job offers.