How Do I Take Photos at Dusk and at Night?
Controlling aperture and shutter speed are the keys. Aperture is the size of the lens opening; you want large apertures to let in as much light as possible. Shutter speed will control how long the shutter stays open; the longer it's open, the more light will reach the sensor. Also, setting higher ISO speeds will increase the "light gathering" ability of the camera's sensor. The Nikon D90 and D5000 have a night portrait mode that will set the camera and fire the flash for photos of people at night.)
If you're using a VR (vibration reduction) lens, activate the VR. But if you're using a tripod—which is a great idea for night shots, as it will stabilize the camera during long exposures—be sure VR is turned off. Shutter speeds that are likely to yield the best results: 1/15, 1/8, 1/4 second or longer—and you'll need VR or a tripod for those.
For more advanced night shots, a tripod is necessary. Long exposure times—one, ten, even 30 seconds—combined with small apertures (for great depth-of-field) will produce dramatic light-trails and reveal details.