A valid passport is essential. Note that some countries require a certain period of validity of the passport. The identity card can be useful as additional identification. As a rule, both should still be valid for at least half a year. You can usually apply for a new passport or ID card at a citizens’ office responsible for you. In some cities, this can now also be done digitally, but an on-site appointment is often necessary. Particularly in larger metropolitan areas, you should apply for this ahead of time, as the processes often take a long time here due to overload in the citizens’ offices, and appointments are hard to get.