I need to make a cake for May 31st with New York buildings on it. Can anyone give me an idea as to how to do this. Thanks.

If you want like "cake topper" buildings, then you can make them "solid" with RKT and cover in fondant then paint on the windows/designs, or you can make them hollow with gumpaste or fondant (depending on drying time of your fondant) and stand them up.
I personally would go with the gumpaste... just roll it out nice and flat. Cut out windows and such for the building, then is you have something square or rectangular you can use, wrap the gumpaste around that object (standing preferred) and let dry. Depending on your atmosphere and how thin you roll it, it can fully dry in an hour or more.
You can tint the gumpaste, airbrush it, or paint it.
If you dont have anything square to wrap it around you could make panels, so you would need 4 per building (front, back, side, side) and once they dry fully you can attach them via melted chocolate (what I use) or royal icing n such.