I will be in Rotterdam in September and I would like to go to Amsterdam. I think the cheapest and easiest way to go there is by train? I already checked ns.nl and apparently it costs 15,20€ one way. Do I have to buy the tickets in advance to get this price or can I buy them at a counter/vending machine at the station? I ask because I am not surewhen I am free to got to Amsterdam. I read something about an OV-card. Do I really need one?
Thank you :)