Take a bus, if not during a major rush hour. I go to Newark for most of my flights, and they are easier and quicker than taking cab or train. Takes 20 minutes with no traffic, 45+ with traffic)
Coach has direct buses to Port Authority, Olympic trailways, and Airlink: http://www.goairlinknyc.com/index.php
All are about $15 and very easy to find in the airport if you just ask. I take the Coach and you just pay the bus driver.
Here are ALL other options (bus, train, cab) if that doesnt work for you. http://www.nysubway.com/airport/newark.html
Edit: whoops I thought you meant the airport, but did you mean the city, or airport? If you meant the city, then just take the PATH train out of Penn Station. Very quick!
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