If some form of artificial light is needed, the most convenient choice is of course your in-camera flash. Its power is enough to light subjects up to 3 or 4 meters away and it can be perfect for filling in dark shadows when capturing images in harsh sunlight.
It is less suited for direct flash however, because it emits a hard direct light that could be softened a little by taping a small piece of tracing paper over it.
Some cameras have a hot shoe or a synch socket to fit a portable flashgun. These are available in several types, but some cameras only work with dedicated "own-brand" flashguns. If yours offers TTL (through the lens) flash metering, this will be the most accurate flash illumination. Most have some sort of automatic mode however. If you set the camera's aperture on the flashgun it will produce exactly enough light to ensure a perfect exposure. The sensor on the front of the flashgun measures the light reflected back from the subject, cutting the output when sufficient light has been emitted. If your flashgun has a swivable head, it lets you bounce light off a white ceiling or wall to give a nice soft modeling light. For even more creative effects, use the flashgun off camera and connect it with a synch lead. If you don't have a hot shoe or synch socket you could buy a slave flash unit which fires when it senses the light from the in-camera flash.