Make sure the subject is in the center of the focus area and press the shutter button halfway. Then you can reframe your picture before you press the button all the way down. See Taking Photos Using the LCD Screen for details.
Make sure you are at least 20 inches away from your subject (or 4.3 inches in Macro mode).
Be sure to hold the camera in the proper position when shooting, with your arm close to your body. Use a tripod for night scenes and other low light situations.