A written petition (letter or pre-printed form) signed by both spouses stating you want to divorce. You do not need to explain your reasons for wanting a divorce.
A document specifying all the consequences of the divorce on which you have agreed (agreement on how to deal with the consequences of the divorce): liquidation of the marriage, distribution of the pension fund, child custody, maintenance payments, division of property, etc.
The court may also request other documents.
Occupational benefits in the event of divorce – Guide for married couples and registered partners (available in German, French and Italian).
Federal Office of Justice: forms for written documents (available in German, French and Italian), including forms for a joint application for divorce, form for applying for legal aid, etc.