http://www.mta.info/mta/airtrain.htm
http://www.mta.info/lirr/html/ttn/pennstat.htm
At Penn Station find the LIRR to Jamaica.Take the LIRR to Jamaica and follow the signs to AirTrain. It'll take about an hour (35 min and 10 min actual riding time) and cost $13 per the website.
You won't be checking luggage, just drag it along with you.
penn staion to JFK