can you please help me get from LGA to Penn station. I need to catch the train out to Long Island. I do not want to take a taxi.

thank you.

Whoa....Laguardia is on Long Island. Where exactly do you have to go? And, when you say you don't want to take a taxi, does that me you are looking for the cheapest way?

heading out to Islip airport, where I will pickup a rental car. I am flying out of ISP.

want the cheapest way.

LGa is in queens which is art of Long Island. It would be very out of the way and (not easy)to go into the city and then turn around back to LI.
Take a look at the MTA/LIRR (Long Island railroad station) website.
Maybe you could get a bus (MTA/Queens bus) to a local LIRR station (there's one in Flushing, I think, close to LGA) and then take train from there to Islip.
Queens bus map http://www.mta.info/lirr/
Railroad info http://www.mta.info/lirr/
This is still not a fun trip...

Take a cab from LGA to the Woodside Satation on the LIRR. Many trains go through this station and you might even connect to one for Ronkonkoma, the Islip (Macarthur) Airport stop. Or from Woodside just take the first train going east to Jamaica, where you can change to the Ronkonkoma train.

DO NOT go to Flushing--it is on a different branch and you'd have to backtack to Woodside anyway.

I think you can also take the Q33 bus from LGA to Woodside, but I'd opt for the cab to save time and agony.

Take the Q33 bus at LGA to 74th street in Corona (Queens) and get on the subway. Take the E or F train to Penn Station.

wlbox does not need to go all the way to Penn Station to catch the same train he or she could get at Woodside or Jamaica.

I'd take a taxi from LGA to Woodside or Jamaica and get the LIRR there to Ronkonkoma. As has been said, going into Manhattan is taking you in the wrong direction. What's confusing me is why you're renting a car from ISP when you could easily rent a car at LGA and return it at ISP if that's where you're departing.

Agreed. I didn't realize that Long Island was the eventual destination (Penn Station jumped out at me in the post).

Agreed - if you're landing at LGA (already ON Long Island) simply rent the car there and drive to wherever you want to go. It makes no sense whatever to trek all around Queens, Nassau and Suffolk by bus and train just to pick up a rental car at Islip. (Even if you do take the bus or cab to the LIRR station in Jamaica or Woodside to take the train to Ronkonkoma you then have to take another cab or bus to get from Ronkonkoma to the airport in Islip.)

And going to Penn Station makes no sense - it's the opposite direction to where you're going.

Perhaps if you tell us more (what you're doing with the car after you get it at Islip) we could provide more useful overall advice.

Do you think this person is ever coming back?