Airport to the Train Station
I have a flight from Brussels Charleroi to Nimes. I know that there's the train from Nimes to Montpellier, where I have to go. But I have never been in Nimes before so I don't know the way from Airport to station.
As a first time traveller my suggestion is to take a taxi from the airport to the train station. There should be taxis outside the airport on arrival. The cost will be about 25 euros.
If you want a little adventure you could take the airport shuttle. The schedule can be found here. It is in French but you should be able to read it. The cost to take the shuttle is 5 euros.