Acute arrest: stay silent, demand a defence lawyer, have relatives notified.
The first hours after an arrest decide a great deal. From a legal perspective three principles apply. First, use the right to remain silent, no one has to incriminate themselves (Section 164 StPO). Second, demand a defence lawyer at once, in detention the defence is mandatory and therefore required by law (Section 61 StPO). Third, insist on the right to have a relative or a person of trust notified (Section 171 para 4 StPO).
As a German national you may also request that the German embassy or consulate be informed, which follows from the Vienna Convention on Consular Relations. Provide your personal details, but make no statement on the matter before a defence lawyer has spoken with you.