From Hanover Airport, take the S-Bahn line 5 to Hanover Hauptbahnhof (main station). It runs every 30 minutes (between 03:36 am and 01:06 am). The departure is always 6 and 36 minutes after every hour on the hour, the journey takes 18 minutes. The S-Bahn station is located directly below Terminal C.
If you wish to travel directly from Hanover Airport to the Hannover Congress Centrum, change at the main train station to bus 128 or 134 in the direction of Peiner Straße and get off at the Hannover Congress Centrum. The journey takes 10 minutes.
The general information page for Hanover Airport can be found here.