As Kenan says, there tend to be different requirements regarding German in each university, however for a basic overview of the process and requirements you can check out this website:
****://***.fluechtlingshilfe-castrop.de/information-for-refugees/study-in-germany/
The website has a lot of good information: hover over "Refugees" in the top bar for a list of other resources. Specifically in regards to studying you can find a list of unviersities that offer courses for refugees are guest students (as well as a map of these Unis), official information about legal requirements for studying in Germany, and instructions about how to prepare/apply for study
By the way, would you like to stay in Kassel to study?