When you arrive in Amsterdam you will find (if still there) one ticket machine in the area where you collect your luggage. Which is very nice if you have to wait for your luggage to arrive, but not worth waiting any longer than you need for your luggage.
When you leave the secure area you follow the signs for the train and you arrive in the main arrivals hall. In there is a series of ticket machines, at least 6 individual ones and around the area are ticket windows. Just be aware that buying from a ticket window is €0.50 more expensive than from a ticket machine.
And these days the tickets from the machines are already €1 more expensive than home prints or when traveling on the OV chip card.
When you are sure that you will travel that day, you can buy tickets online. The tickets for the Dutch trains are not timed, so if you have a ticket for one train on the day you can also use an earlier or later train, but just that one day. Only when you buy tickets for the Thalys you can only use it for that one train.
But I agree with one of the other answerers, consider buying in OV chip card. (Also available from the ticket machines and the ticket windows.) Put at least €20 on it and you can use any train (except the Thalys.) Although some fast trains like the one to Rotterdam/Breda and those to Germany, require a surcharge, which you can also put on the OV chip card.
Remember that you need at least €20 on your OV chip card for train travel and at least €4 for bus, tram and metro.