If you want to travel to and from the airport using public transport, you can take the AirportExpress which connects directly with Dortmund Central Station. If you choose to take the AirportShuttle, this takes you directly to Holzwickede station. You can travel to both stations on the Deutsche Bahn's local and intercity trains.
If you are planning to purchase your ticket online, please refer to the Deutsche Bahn website. This only pertains to public transport and excludes shuttle tickets which can be purchased on the bus.