Night time shots of city skylines and buildings are usually best taken at dusk when there's just a little bit of light left in the sky. They'll tend to look flat if shot after dark - try both and see what you think. Make sure to turn the flash off too! One piece of equipment that you really can't do without is a tripod (and a cable release or remote release, OK - that's two). With this, you'll have long shutter speeds and end up with blurry shots without a tripod.

Another good thing to use is a bubble level that goes in the flash hotshoe, this way you can easily tell if your camera is level. It can be very hard to tell if it's level or not by looking thru it in the dark, and (unintentional) tilted horizons don't look good! All of this will be good for daylight too, a tripod will make you slow down and really make sure your composition is what you want.