Coming from Newark, NJ Airport:
There are several bus options to get to and from the NJ airport from the NY Penn Station or the Port Authority Bus Terminal, but the easiest is taking the AirTrain system. It takes you around the airport and to the NJ Penn Station. It costs $5.50 to get off of it at the NJ Penn Station. From the NJ Penn Station you can buy a ticket on either the NJ Transit system or Amtrak to get to the NY Penn Station, which connects to the Blue A, C, E subway lines and the Red 1, 2, 3 subway lines.
Coming from LaGuardia Airport:
You can take an express bus directly to midtown for $13 on the NYC Airporter.
Coming from JFK Airport:
AirTrain JFK is the cheapest way to travel to, from, and around Kennedy International Airport. It serves as a shuttle between terminals & parking lots, and connects to several subway lines. It costs $5. You pay at the end of your ride when your about to get onto the subway (which will cost an additional $2.50). There are plenty of machines you can use to purchase your tickets using cash & credit card, and you can purchase a metro card to use for the rest of the weekend at the same time.
If you have a lot of luggage and don’t want to deal with transfers on trains, there is a bus service that goes directly from the airport to grand central. It is called NYC Airporter, and you can either prepay for your tickets online or after you pick up your luggage at the terminal. The buses run about every half hour, and they stop running at 11:30pm. Here is the website: http://www.nycairporter.com/