Tari, if you're staying in NYC overnight, the best thing to do is to call the hotel and ask where the nearest parking garage is as street parking is going to be really difficult. Subway is the best option to get around much of the city except some parts of the upper east side. The #2 and/or 3 train will get you around between WTC and Empire State Bldg very easily for example with a short walk included. @2 is an express with limited stops, and #3 is a local with more stops but both would serve you well. Cabs are ok, but a bit costly. The buses are really a slow mode of travel.

If you know where you're going to go specifically, you can use www.hopstop.com to plan out your travel in city. It'll give you subway, bus and walking directions to anywhere you want to go. Excellent site that I use frequently when I work in NYC. One word of note about the subway if you decide to use it....it's absolutely sweltering down there these days.

Can't help with getting to Yankee Stadium. As a Mets fan, I've only been there twice in my life. It's not a fun drive from Long Island where I live so if I'm ever going to go there again, I'm going to make sure I'm a passenger.