Take the bus to LGA,NY airport?

Yeah, go down to the Port Authority Bus Terminal on 42nd Street and and take the New York Airport Service, which is an express bus, to LGA. The one way fare is $12.

If you're on the East Side of Manhattan, you can take the subway to Grand Central and take the bus from there to the airport. The fare is the same as if you were going from the PA Bus Terminal.

Good luck
Native New Yorker