Geert,
Have you tried going through the tutorial here: http://community.thingspeak.com/tutorials/arduino/send-data-to-thingspeak-with-arduino/
I would recommend starting with one of the simpler examples in the ThingSpeak library. See tutorial here: http://community.thingspeak.com/2015/10/official-thingspeak-library-for-arduino-and-particle/
Once you are able to get one of the simple examples working with your Arduino, you will be able to build your project using ThingSpeak.