You will find the date of the test on the application form which will be sent to you together with the application for admission. This can be used to borrow books from the department's library. To purchase your ISIC, please do not forget to bring along your current student ID and a passport photo. There is no such thing as a strict time-table. To know exactly what academic achievements will be expected of you, please consult the study regulations and examinations regulations.
Students have several choices.
With regard to what kind of seminar courses and lectures you should attend please ask your programme coordinator or the respective subject's course advisers for help. You will find the times of the course advisers' consultation sessions in the university catalogue. Virtually all subjects offer some kind of introduction to their courses of study. In some subjects, you might have to register for specific courses. Each semester, the university publishes a Personen- und Vorlesungsverzeichnis which contains the names and addresses of staff members, the various departments plus course catalogues of all the subjects that are taught in Mainz.