The library's holdings are shelved in classified order, that is, arranged according to subject areas and contexts. (How the subject areas are built and how subordination and superordination relate can be followed in the classification scheme).
Each book bears a call number which consists of letters and ciphers (e.g. HI810 N436). This code represents the textual affiliation to a subject and thus the position on the shelf.
In this example the book is shelved in the informatics section, deals with networks, teleprocessing and telecommunication and can be found under N in the alphabetic sorting.
Bielefeld University Library keeps the the majority of its holdings in open stacks in the departmental libraries. In order to find the literature you need you can simply browse the shelves in the subject area you're interested in. Or, if you already have a certain title in mind and you have searched the library catalogue for the call number. Either way you will need to refer to the library's site plans for orientation. You will find these in print at the circulation desks or online in the subject portals.