As soon as possible after your arrival you should get in touch with the academic international office of the university and the advisors for international affairs at the Studentenwerk.
They often have useful tips and can support you with help and advice! Many universities help international students to acclimatise by arranging help with administrative aspects and giving a general guide to the university and its facilities.
Is there a tutor resident in your student accommodation? If so, you should get in touch with him/her, too!
Have a word with the students' council at your faculty as they can help you with choosing courses and putting together a timetable. Several courses also offer introductory events during the first term. Here you can get tips on organising your studies and make contact with other students.
Another important point of contact for students is the AStA (student parliament). Students have the opportunity to voice their interests via this politically oriented board of higher education. The AStA also offers help and advice in several areas. There is often a contact person especially for international students.
The services of the AStA differ from university to university; apart from social and legal advice they also offer cultural activities and cheap learning aids.
Besides the AStA there are other student associations - evangelical and catholic university communities, several clubs and political university groups.