It is highly recommended for foreign applicants for a place of study in a German-taught study course to attend a German language course before leaving their home country. Sometimes these are offered by the home university or the German cultural institutions (e.g. the Goethe-Institut, the DAAD) in the home country. The addresses of such institutions can be obtained from German embassies and consulates.
Furthermore, the Deutsch-Uni Online is a webportal for learning German. The DUO German courses offer all the necessary knowledge - covering language, knowledge about culture and everyday life, and subject-specific skills. Also, you can check or train your German Grammar (structured according to the Common European Framework of Reference for Languages) with the Deutsch Akademie.
Moreover You have the opportunity to participate in the International Summer German Courses at Bielefeld University. They take place every August and You can choose among different language levels and themes.