At WMU, both on campus and off, there is a bus system that will take you to your destination. There are stops in local neighborhoods where students reside, and the bus will make a number of stops all over campus. The bus system is most used by freshmen and commuters who do not have a parking pass. There are also a number of taxi services around the city that can take you pretty much anywhere. Usually, there is a flat rate per person, but for some of the taxi services, they charge by distance.
Kalamazoo was founded as a "train city," and the Amtrak, which sits downtown, is a very popular mode of transportation between the eastern side of the state and even Chicago. In terms of more localized ways to get around, you can expect to see the majority of people at Western walking or riding their bikes to class each day, as it is the most common form of transportation. Western's campus is rather large, so plan accordingly.