It is beyond the scope of this guide to teach lighting technique. There are many books, classes and DVD's on the subject and I'm sure most of them do a better job of explaining how to light a portrait better than I could.
However, the first place to start when thinking about lighting is to avoid your on-camera flash.
The flash built into your camera may provide enough light for your photo, but that is all it will provide. Because the on-camera flash gives off a burst of direct, unfiltered flight, it makes for a very flat image. It is fine for a snapshot. It will not help you create a beautiful portrait. The same is true for flash units attached to the camera. If they are simply pointed at the subject from the top of your camera, you will have a harsh, flat, unflattering light.
Some of you may object and say the on-camera flash can help provide a "fill light." While this is true, it does not change the fact that the fill light will be from a single flat point. If this is your only option to get the photo, then of course you should take it.
If all you have for lighting equipment is the flash on top of your camera, try turning it off and working exclusively with natural light.