It's Grand Central Terminal and there is no subway service to LGA.

There are many ways to get between EWR and LGA. The most direct is the most expensive and that would be a car service. The least expensive is a pain in the butt but here's how to do it:

From EWR, get on the AirTrain from whatever terminal you're in and take it past station P4 to the Newark Liberty Train Station (rail-link). From there you can take New Jersey Transit (or Amtrak, which will cost more) directly into New York Penn(sylvania) Station. Make sure you're buying a ticket to New York Penn Station because there is Newark Penn Station too.

You can buy your NJT Ticket from a machine in each terminal just at the base of the AirTrain escalator. You'll want to do this because there is a $5 or $6 fee to ride the AirTrain to the rail-link stop (Newark Liberty Train Station) but this fee is waived with a NJT or Amtrak ticket. Thus you'll want to have your NJT ticket in advance of the AirTrain ride.

Once in Penn Station, you'll need to buy a Metrocard for the subway and bus. The fare is $2. You can can take the 2 Train or the 3 Train (the subway) uptown to the 125th St stop (at Broadway). From 125th St (on the same $2 fare) you'll transfer to the M60 bus and take that the rest of the way to LGA.

The total cost will be $17...$15 for a one-way NJT ticket from EWR to NY Penn plus $2 for the subway to the bus to LGA. This is the cheapest option...not necessarily the best.

These guys charge about $28.

You can also take the Newark Airport Express bus to the Port Authority Bus Terminal (W 42nd St and 8th Ave) and connect to the New York Airport Service Express Bus to LGA. Total cost about $22.

Tipsy