Their sites says luggage carts are allowed on AirTrain, so you'll likely have to rent one from a machine, meaning you will need American coins. "Airport luggage carts are allowed on AirTrain, but they are not meant to be put on escalators"

Don't take offers of help from anyone else as there are no official porters and they may rob you.

Here's more on the service: http://www.panynj.gov/airports/jfk-airtrain.html

You want to get off at Federal Circle Station.