See if these links help:
Information on bus services to and from the University of Siegen:
https://www.uni-siegen.de/incoming/siegen/transport/index.html.en?lang=en
Siegen bus route map:
https://ontheworldmap.com/germany/city/siegen/siegen-bus-map.jpg
Bus timetables:
https://www.vws-siegen.de/fahrplan/fahrplaene/
Call into the Tourist Information Centre (Rathaus, Markt 2, 0900-1700, Mon-Fri) and pick up a map of the city (or seek out one from a bookshop). Find places on it that are important to you and mark them on the map, then use the bus timetables to plan out theoretical journeys between them.
Simply saying words out loud can help you to remember them. (Don't worry about it looking as if you're talking to yourself. With nearly everyone using mobile phones and Bluetooth earpieces these days, it looks perfectly normal!). So, for example, say "Bushaltestelle" out loud when you see Siegen's main bus station, and look around as you do so. It should help you remember both what it is and where it is.