A civil lawsuit is very different from a criminal lawsuit. The police and the public prosecutor's office do not carry out any investigations.
You yourself gather the necessary evidence. The beginning of a civil lawsuit is usually the so-called written preliminary proceeding ("schriftliches Vorverfahren"). It prepares the main appointment.
Here, the plaintiff first states the claim (for example, for payment of compensation for pain and suffering) and names the evidence (for example, witnesses, medical reports, photographs) on which they base the claim. The opposing party then responds, attempting to invalidate the evidence, for example, by naming its own witnesses. This written preliminary proceeding can take several months until it gets to trial.
In a civil case, you need to comply with many formalities and gather all the evidence yourself. This is why it is important to have the support of a lawyer. It makes sense to seek in-depth advice before filing a lawsuit. If you are on a low income, you can apply for a so-called "Beratungsschein" (advice voucher) with your district court. If your application is successful, the state will cover the costs of out-of-court legal advice. If the case goes to court, you can apply for legal aid. This covers the costs of a court trial – including court costs and the costs for your own lawyer. You apply for legal aid with the competent court.