I'm also travelling from Belgium to JKF next week.
3 solutions:
If you have a lot a luggage, take a shuttle airport. You can ask for one at info point in JKF (ground transport information).You can ask them to call a shuttle for you. $20 for the price
Airtrain + LIRR : $15 the fastier but you have to carrier your luggage yourself...
Taxi: the more expensive and not the shorter in time... $50 + toll + tips
We chose airtrain+LIRR for next week, we have wheels on our luggage ;-)
JFK to Manhattan