I'm a DAL guy, and sometimes commute into EWR to get to JFK. There is a van service between EWR and JFK/LGA. I've paid $15 in my uniform. Just go down to baggage claim and tell the folks at the Ground Transportation desk what you want to do. An alternate is the Jersey Transit train from EWR to Penn Station, then the LIRR to Jamaica and a cab to LGA. That will cost $30-40.