To be enroled as a Student (German or international) it is compulsory to have a valid health insurance. To be able to register a document from one of the German health insurances is necessary.
If you are an international student and the health insurance from your home country is not valid in Germany, you have the following options:
If you already want to stay in Germany before the term starts, you have to cover this time with a private insurance. There are different health insurance providers offering such an insurance, the cost for a policy depends on the coverage and can vary considerably.
With the start of the Semester you can get statury health insurance by one of the German Providers. Students pay approx. 80€ per month. It will cover most health related cost.