The best way to get around Amsterdam or to/from the airport is the Amsterdam Travel Ticket. They’ve finally figured out a system that I can support. This card gives you unlimited travel between the airport and Amsterdam central station, plus it gives you unlimited travel with the GVB (local busses and metro) or busses 197/ N97 that go to and from the airport as well.
You can buy the pass for one, two, or three days ranging from 15€/20€/25€. If you are staying at the airport or if you are staying near the Sloterdijk train station this is really a great deal.
It can be bought at the airport or even within the city. At the airport it can be bought specifically at the NS ticket desk or these various locations throughout the airport and Amsterdam city. You can also buy it online and then are required to print it out and bring idea with you to use it. But I find this cumbersome as the card you’d get at the airport would easily fit in your wallet and it’s the same shape and size as a credit card.
This is a pretty great deal even if you might not use the transport system too much in the city. Keep in mind a single ride on the tram is €2,90, and the costs to and from the airport alone are 10€ return. Doesn’t take long to make this really worth it.