Hi,
I live in NYC now, but I am not a broker. I can give you some info about finding apartment here.
Right now NYC area has a fairly tight rental market, which means apartment availability is pretty low. Landlord has been demanding for good renters, so it's unlikely you can lock up a place until you come.
Try to find a friend, relatives, former classmates, or whoever you know and stay in his/her/their place when you first arrive (if the person let's you). You can stay in hotel/motel as well but it's not cheap. Use the first 2~3 days searching for apartments. Try to arrive at the beginning of the month, when there are more availability.
If you plan to use a broker, be aware their tactics to push you into a place you might not like. Normal broker fee is 12~15% of annual rent. New York Times has RE site that has most rental listing across NYC & NJ. Those listings are posted by brokers.
http://www.nytimes.com/pages/realestate/index.html
If you try to search on your own, try Craigslist.org.
http://newyork.craigslist.org/hhh/
Some of listings are bogus or false, and be aware "no-fee listing" posted by some brokers.
Hope this helps.