Are you arriving at Groningen airport?
If yes, you can take bus service 2 which departs semi-hourly at minute 19 and 49 and takes 37 minutes to the Groningen train station.
From there you have semi-hourly train connections.
At minute 16 with Intercity train to Rotterdam and change of trains at Amersfort; journey time to Amsterdam Centraal 2 hours 14 minutes.
At minute 46 with Intercity train to Den Haag and change of trains at Lelystad Centrum; journey time 2 hours 7 minutes.
The train to Den Haag also stops at Amsterdam Zuid after about 2 hours; from there you can reach the center of Amsterdam by Metro service 51 in 15 minutes.
Routeplanner National Railways
http://www.ns.nl/en/travellers/home
Station Groningen - Amsterdam CS
Singleticket
