A Google search gives a few possibilities, and I'm not sure how these differ or which would be appropriate:
- Einführung
- Einleitung
- Bekanntschaft
- Vorstellung
Einleitung would be the most common, I think. Einführung (which can also mean insertion, depending on context) is not wrong, but usually refers to something broader, like an introductory course or similar. Your last two examples (i.e. Bekanntschaft and Vorstellung) refer to persons being introduced or getting to know one another.
Without knowing the type of document you want to write, it is difficult to decide. For technical papers frequently something like Motivation (same meaning as in English) appears at the beginning, to describe starting point and to problems to be addressed in the paper.