Metro card -- works on bus or subway. You'll want the seven day unlimited ride one. Available at any subway station (use your credit card at a vending machine and just follow the instructions.).
It's very easy to get around New York. The whole place is small enough to walk (the blocks are small and you're on holiday anyway, what's the rush?). Just make sure that the neighborhood isn't dangerous (if it feels "icky" get out!)
I highly recommend the subway (no it's not dangerous as long as you're not acting like a dough-head) or the bus. Until recently, you couldn't transfer from subway to bus and vice versa, but now they have something called a "Metro Card". It acts like a credit card. Get the 7 day unlimited pass. You'll be putting between 20 and 40 rides on that sucker eventually. Or, you can always just buy subway tokens ($1.50 each -- also accepted on the bus). Those you can buy in handy packs of 10. Have your change ready. Both subway workers and subway riders are REALLY SNARKY if you don't have the correct amount of the money.
Taxi ONLY if you have to. The ride in from La Guardia should only be about $20. You can also "taxi share" (12 per person, if there's two of you, don't share, you still have to pay per person -- get your own cab). Make sure you're at the CAB STAND because New York has what they call "Gypsy Cabs". These are NON-OFFICIAL cabs. They look just like yellow cab, but you'll find very quickly they have no meter!!! These are NOT dangerous, but be sure to negotiate your fee AHEAD of time. If you can't guess-timate your fee, get a real cab (at the Taxi stand at the airport). I only Taxi if I'm getting out from a show VERY late (after 11:30) and am by myself and in a dangerous part of town (very uptown or very in the village). Otherwise, I walk or subway.
Speaking of walking, one of the coolest things about New York is that you can (nay, are expected to) Jay Walk. New Yorkers don't give a hoot what colour the light is. In fact, they get annoyed if you stop at a red light (I've had people bump into me from behind). This practice is not nearly as dangerous as it sounds. New York is such an old city, most of the streets are all "one way", so it's actually very easy to dodge the one lane of moving traffic. The only real danger you're in is from being mowed down by a bike messenger (and that's on the sidewalk!)
Overview/Introduction
Before You Go
Where to Stay
Shopping!
How to Get Tickets
Choosing Shows
Cheap Tourist Things to Do
Getting Around
Being Safe
Random Tips/Thoughts
The Barb's Broadway Reviews