Basically, the first point of contact for issues surrounding your studies in general is the academic counselling office of the individual university. Information about registration and all "technical" questions is given by the Registrar's Office. Once you are about to register or have registered, the student unions (university-wide) and the student council (subject-specific) will provide help. Especially the student representatives often organize events for first-year students and info tables that are essential for a first (subject-specific) orientation.
Here you will find a collection of links how to contact the above mentioned organizations: