You can buy tickets in the tram (or bus).
But if you want to travel with the tram more often you could also consider buying an “anonymous OV chipkaart”. The anonymous OV-chipkaart is a plastic card on which an amount or a travel product can been loaded.
The anonymous OV-chipkaart you can buy in the yellow venting machines on train stations for €7.50, using your pin card or credit card. You can recognize the machines suited for this purchase looking for the pink sign:
These machines will also enable you to charge the anonymous OV chipkaart with the amount you choose.
For more information and buying tickets you could also go to the GVB Tickets & Info service at Stationsplein CS.
monday till friday: 07:00 - 21:00
saterday & sunday: 10:00 - 18:00
Entering the tram or bus you need to check-in by holding the card in front of the check-in machine in the tram or bus. Doing the same when exiting the vehicle, you will be checked-out.
NS ticket machine - Taken from the website of NRC Handelsblad
To plan your trip with the tram check: www.9292ov.nl
Also available in English