Motorway A40 runs almost immediately along the Signal Iduna Park, which makes the stadium good to reach by car. Around the stadium there's a lot of parking space available, where you are leaded easily from the motorway. Make sure you have cash with you, to pay for the place in advance.
Behind the Südtribune is train station Dortmund Signal-Iduna-Park, where you get from station Dortmund Hauptbahnhof in less than ten minutes. From the city you can also take the metro (the U-Bahn), for example from Reinoldikirche or Stadtgarten, with terminus Westfalenhalle/Stadion.