Before heading out to get some great images of the city after sunset, it is important to make sure you have the proper equipment. This means that you should definitely have a flexible tripod that can adjust to varying heights; a wide-angle lens; some sort of lens hood or a system for directing the flash outside of the frame; and a game plan.
Urban areas can be dangerous locations in which to setup a camera and take a prolonged series of images. This is not due to the potential for a crime alone, but also because of busy streets, and the fact that people do not see well in the dark. This translates to the need for a photographer to select their target shots and consider the settings that their camera might need to make the images they want.
If the camera has a capacity for custom "presets" a photographer who makes a regular habit of nighttime photography might want to dial in these settings in order to make their work as easy as the tap of a button.
Considering a body of water or a place like Time's Square for some Urban Photography. But the main rule is be careful in urban areas, not only are you out at dark but busy streets can be a problem. Selecting the image ahead of time, or just a general shot of what you would like to capture, would be a good beginning to your shots at night.