If you're shooting just one or two people at a time, try to use the narrowest depth of field possible. This brings the subject into sharp focus while causing the background to melt away in a gentle blur. The easiest way to do that is by using your camera's aperture priority mode and dialing in a small f-stop number.

For group shots, you'll want to set the aperture in exactly the opposite direction: To ensure that everyone in the photo is in focus, set the biggest f-stop number that your camera allows; this will help you achieve enough depth of field to ensure that everyone from front to back will be in focus. The background won't blur as it does when you shoot with a small f-number, but you'll have better luck keeping everyone in focus.

If you position your subject in front of a window, you'll want to overexpose the scene a bit, because your camera's sensor will be confused by the daylight streaming in the back of the shot. Left to its own devices, your camera will tend to underexpose the faces of your subjects. Use your camera's exposure compensation control to start with a value of +1, and then experiment to see what works best.