Bout the only place I know of that you can get a Metrocard in the airport is the Hudson News in the US Airways terminal (near baggage claim), and probably the one in the Central Terminal..
There are several change machines, where you'll be able to change dollars into quarters (you CANNOT use dollars on the bus)...
In any case, everything depends where your hotel is..
To me, it's really not worth it to take a cab to NYC, as you just burn $30 and don't save all that much time.
M60 runs quite frequently, every 5-10 minutes, along with the subways.
Anyway, if your hotel is on the East Side, take the M60 to 125th and Lexington (there is a Duane Reade on the corner where the stop is, and across the street from taht is a McDonalds), then either the 4/5/6 to wherever your stop is.
If it's on the West Side, near Broadway at least, M60 to Astoria Blvd, then N/W to the city... It's not worth taking the M60 to Frederick Douglass Blvd on 125th for the A/B/C/D because of traffic.
If your hotel is wayy on the westside, then you may have to do this, take the M60 to 125th and Frederick Douglass Blvd (there's a Krispy Kreme across the street and over from the stop). Then take the A or D (express to 59th street) down to Midtown.
Ride is very safe, but that's coming from a NY'er who grew up here, and been in some pretty bad neighborhoods. I've met tourist friends at LGA
and taken them on the M60 and some say they've been a little nervous...
Anyway, just don't look completely oblivious to your surroundings... People will be able to tell you are tourists right away, so walk quickly, look like you know what you're doing, and I wouldn't advise taking the M60 to 125th after 11PM...