Passau has a vibrant private housing market. The average rent is about €400 a month for one bedroom. Landlords usually require a two- or three-month deposit when you move in, which will be returned to you at the end of the rental period, assuming you leave your room in good condition.
If you are looking for shared accommodation and student flats, the most important web portal is WG-gesucht, which is also available in English. WG stands for "Wohngemeinschaft" - flat shares or house shares.
To find private accommodation, check the notice boards on campus (Philosophicum, Faculty of Economics and Business Administration, Refectory).
The Wednesday and Saturday editions of the local newspaper 'Passauer neue Presse' (PNP) have an extensive accommodation section. Their classifieds are online, too (see above under 'housing websites').
Please read through the housing advice page before you sign a tenancy agreement. Contracts and tenants' obligations vary widely. If you are renting for the first time in Germany, some common clauses may surprise you.