The NYC travel forum covers this extensively, but I'll give you the quick and dirty:

The cost of the ticket is about $15, including admission to the airtrain. The ride from NYP to the EWR station is about 30-40 mins, but waiting for the airtrain to arrive and poke along to the terminals can take another 10-20. So I suggest catching a train that is one hour before you wish to arrive at the check in counter, or 2-3 hours before your flight. At off times of the day a cab is faster, but at rush hour this is by far the fastest way to the airport.

At NYP, the trains to EWR are very easy to identify because they will display "EWR" on the tv monitor announcing the trains/tracks. All of the trains that stop at EWR are direct to the rail-link terminal, no connection or switching of trains is required. Also, be sure not to get off at Newark-Penn Station, but rather wait until the next stop which is EWR airport.