First of all: The state-funded financial aid programme BAföG (= Federal Education and Trainings Assistance Act) is the best way to finance your studies. The BAföG service office as well as the student services (Studentenwerk/Studierendenwerk) at your place of study will provide detailed advice. Attention: If you have a residence permit for study purposes (§ 16 AufenthG) you are not eligible for BAföG!
Students in the summer semester 2016 had an average of 918 euros per month, according to the 21th Social Survey of Deutsches Studentenwerk (DSW). This long-term study analysed how students financed their studies.