Documents to help me get a Schengen visa to Germany [duplicate]

If your girlfriend promises to pay for your housing and living expenses, that explains where you will stay in the Schengen area and how you can afford it. It does not make it any more likely that you have the intention to leave the Schengen area again. For that, you have to explain your economic situation in your home country, the best explanation would be a stable and well-paid job.

Germany has a legal document called a Verpflichtungserklärung, which is a formal promise to repay the German state for any costs if you overstay, voluntarily or involuntarily. If your girlfriend wants to file that with the municipal government, she will have to show her own residence status and income. It may be necessary for her to post a bond if she does not earn enough. (Her repayments would not be limited to that bond if you overstay. Deportations can be quite expensive.)