To get to the exhibition centre, take tram no. U78 MERKUR SPIEL-ARENA/Messe Nord or U79 Messe Ost (Exit at MesseOst/Stockumer Kirchstr.) or bus no. 722 (Exit at Messe Ost or Messe Süd/CCD). There are two major transport hubs where you are likely to change: Hauptbahnhof (central station) and Heinrich-Heine-Allee. Nearly all destinations in and around Düsseldorf can be reached from these stations.
For most trade fairs you can use your pre-purchased entrance ticket which is also valid for free local transport to and from the Düsseldorf Exhibition Centre on the day of your visit, provided you remain within the area covered by VRR (2nd class, zone D).
Moreover, there are several trade fairs where your entrance ticket also covers the neighbouring VRS area (which includes Cologne and Bonn).
You will find suitable information on your entrance ticket.