You are invited by a German host but do not pocess your owb financial means? Than you would have to hand in a so called‚ formal letter of obligation‘.
It is essential that you have the original document at hand when you come in to apply for your visa. The fomal letter of obligation also has to provide the correct reason: e.g. visit, language course etc.
Your host has to send the formal letter of obligation directly to you (we advise to do so via courier so the document does not get lost). The document should not be send to the Embassy!
What it exactly entails and what your German host has to do in order to receive the necessary document can be found on our German Website.