1. Talk to your health insurance; they often have scheduling services
2. Talk to the Kassenärztlicher Bereitschaftsdienst of your state, e.g., Bavaria has this on offer: https://www.kvb.de/service/patienten/terminservicestelle/ter... If you don't know where to head to, call 116117. Easiest way.
3. Go to your GP - he can guide you through this or can forward you to a psychiatrist
4. Use the online service from the association of psychotherapists: https://www.psychotherapiesuche.de/pid/ersteschritte
If you have suicidal thoughts, search for a local Krisendienst Psychiatrie (https://duckduckgo.com/?t=ffab&q=krisendienst+psychiatrie&ia...), they can help you immediately.
The most important thing that OP has to learn is that he is not alone, there are professionals out there that can help you. But you have to make the first step.