You must be a matriculated student, or be in the gap year between your bachelor and master programs. We need to have confirmation of your matriculation, or a document confirming your intention to begin a master's program of study.
And one more important point: You should be creative and able to work well in teams, with the skill to communicate fluently in German and English. What's more, you must be familiar with Microsoft Office applications, and – if possible – already have some kind of job-related experience.
Specific details are included in each vacancy notice. The contact person named in the vacancy notice will also be happy to answer your questions.