There are some tips
here on using the flash as well a a few other articles listed
here. The first things that comes to mind for the dark images would be a.) the subject was outside the range of the flash and b.) you might possibly have blocked the flash with your hand (I've been known to do that :-/). For red eye, try to keep the subject from looking directly into the camera/flash AND perhaps provide a little extra lighting to help minimize/eliminate it.