In order to get a visa you must be able to prove that you can finance your studies. At least the first year of your studies must be completely financed.
The German authorities assume a certain minimum amount which you must have at your disposal each month. At the moment this lies at 670 Euro per month which comes to a total of 8,040 Euro for the first year of studies. However it is possible that proof of a higher amount might be asked for. Starting from autumn 2016 this figure will rise up to 8,820 Euro per academic year.
Proof of financing can be shown in different ways. It is imperative that you enquire at the German Embassy in your country in what form the proof should be shown!
Do not underestimate the proof of financing. It is more than just a bureaucratic hurdle; you should make sure that even after the first year of your studies you have a certain amount of financial support to fall back on.
For a long time it was relatively easy for students to find work, but this situation has changed due to the long term mass unemployment in Germany. Work is now mostly restricted and financing your studies completely by working is hardly possible.