first of all, welcome to our community!
It's a well known problem to find a room in big cities in Germany - especially if there is a university with lots of students looking for small and affordable accommodation.
You probably already know the usual ways to search for a room, right? Unfortunately we can't do much more than providing general information about how to find a room.
You can try the following websites. Some of them are in German, but it should be easy to understand:
I already set the filter to "Paderborn", but you can adjust them to your needs of course.
If you contact WGs it's always a good idea to send a short self-description along with it. That way they can get a better impression of who you are and it increases your chances to get invited to an interview. When I was looking for a room that definitely helped me to stand out from the others in comparison to an impersonal mass mail.
Another cool project that you can try out is "Wir heißen Flüchtlinge willkommen":
You can sign up and if you are lucky they will match you with a WG offer in Paderborn.
Good luck with your search and best regards,