Thank you very much for your reply, very helpful!
One question: in order to go from the airport to the train station (hpf), it says that it is only 12 minutes. But how do I get there? Is there a bus or something outside the airport?
The airport has itself 2 train stations:
Düsseldorf Flughafen Terminal
The Terminal train station is directly at the terminal building. The airport (Flughafen) train station is connected by the SkyTrain (a monorail) with the terminal building.
If you use the query as suggested and check out the details for a connection DB timetable will tell you all you need to know.