In order to enrol as a student, you will need a valid health insurance. EU citizens can provide their European Health Insurance Card (EHIC).
If your home country does not have a social insurance agreement with Germany, please present your health insurance to one of the public German health insurances (e.g. AOK, TKK, hkk, Barmer GEK). They will check whether your foreign health insurance is valid in Germany and provide you with a written confirmation accordingly. You will need to hand in this confirmation when enrolling. If your health insurance is not sufficient for your stay in Germany, you must take out a public German health insurance of your choice immediately. You will not be able to be enrolled without a valid health insurance!
Please note that a change from public to private health insurance is only possible up to three months after you have enrolled at a university/signed for the public insurance.
More information on health insurance in Germany for incoming visiting students can be found here:
For international full-time students: http://www.krankenkassenzentrale.de/wiki/incoming-en#students