Trains are regularly going to Amsterdam Central Station from the airport. It’s generally about 20-25 minutes by train with several stops along the way.
With 25 minutes from the airport to Amsterdam and 25 minutes back from the Central Station to the airport, you are looking at about 1 hour in traveling time. However, from time to time there are delays. In most cases, the trains in the Netherlands are reliable but there are always some circumstances that can pop up (maintenance issues, weather, etc.)
Train tickets can be bought inside the arrivals hall at Schiphol airport. There might not be a train exactly when you are ready so you might have to wait around for 15 minutes for the next train so include waiting time in your calculations.
The train stop is Amsterdam Central Station. From here you can walk directly to the Dam Square (about 7 minutes) but I’m sure you’ll want to stop and take some pictures and take in the scenery right away before you see the main Dam Square area.
Tickets roundtrip (Airport to Central Station and back) are generally around 7.80 euros per person unless you have a discount card.
Amsterdam is generally considered a smaller type city but if this is your first time, then it will feel like a big place. Most common travel within Amsterdam is by Tram or if you are a local then also biking is very popular.