I am hoping to go to NYC soon and want to know the top places to visit while I'm there. I am obsessed with Gossip Girl and want to go to those locations in the TV series.
Someone's also put together a definitive list of all the locations they could find in the series, together on one map. It's amazingly detailed, and so anywhere on Manhattan that you happen to be, you're probably near a location.
However rather than wander aimlessly trying to see each one, your best bet is to take one of the several tours available specifically for this, from the companies mentioned in @Karlson's answer.