Newark Airport to Manhattan, NYC?

The two easiest are the Airtrain to the NJ Transit commuter train to Penn Station, for $12.50, or the Coach/Olympia bus to the Port Authority Bus Terminal for $15.

The cheapest is the local #62 NJ Transit bus to the Newark train station for $1.50, and then the PATH train to WTC or 33rd st for $2.25.

The other people gave you good answers. I'll just add that you can go to the EWR website and look for "ground transportation" for options.

The easiest and probably cheapest way is to take the AirTrain monorail from the airport to the New Jersey Transit station that is nearby. There, get on any New Jersey Transit train heading into Manhattan--they will take you to Penn Station in midtown.

The trip will take about 40 minutes and will cost about $12.50.

For details:
http://www.panynj.gov/airports/ewr-airtrain.html
http://www.njtransit.com