Graduation in geography is possible with a completed university degree (Diploma Degree, Master Degree, Teaching Degree, Magister Degree). Details can be found in the promotion regulations of the Faculty of Mathematics and Sciences or the Factulty of Arts and Humanities. Foreign PhD-students have to submit a proof of their language skills: DHS2 or TOEFL.
If you want to promote at the Department of Geography (Bonn), firstly you have to get in touch with a possible advisor. The contact details of the professors are available here.