You can buy from the yellow kiosks using card or coins, or from the service desk if you want to use notes, there is an extra charge for that though.
You have to validate your ticket at the gates which are located at all entrances rather than on the platform.
Most of the trains leave from platforms 14a/14b/15 so you'll want to have a look at the screens, note that the trains are not just going from AMS Centraal to Schiphol but rather just go through Schiphol on their way somewhere else, you can always ask any of the guys wearing an NS jacket or at the information desk if it's easier.