First, you will definitely need a tripod. Second, you'll need to have the camera in Manual Mode since you'll be setting the aperture and shutter speed. Third, turn the AF and VR (if your lens has it) to OFF. Forth, have your camera set to its highest resolution and use ISO 100. For exposures up to 30 seconds long you can use the camera's self-timer to release the shutter.
I use and recommend this site for night exposures:
http://www.calculator.org/exposure.aspx
I chose the Scene 'Distant view of city skyline or floodlit buildings' and ISO 200 for these:
http://www.flickr.com/photos/drifter45h/4048051455/ 100mm lens @ f11, exposure of 30 seconds.
http://www.flickr.com/photos/drifter45h/4048796836/ 200mm lens @ f11, exposure of 30 seconds.
I used ISO 200 because that was the ISO of the film I had in my camera. If I had loaded ISO 100 film my exposure would have been f8 for 30 seconds or f11 for 60 seconds. In a previous answer to a similar question someone said that the exposures I used with film would result in overexposure with digital - but no proof was shown. It should be easy for you to determine if this is true and, if it is, then just reduce exposure in 5 second increments until you get the results you like.