First, turn off both your computer and monitor and unplug their power cords. Check your connection type. For the best quality, use a DisplayPort cable (not included). For better quality, use the included HDMI cable. For good quality, use a VGA cable (not included).
If you use VGA, you must also connect a 3.5 mm AUX cable from the monitor's AUDIO IN jack to your computer to get sound. Reconnect all power cords, then turn everything back on. The monitor should switch to the correct source automatically. If it doesn't, you may need to manually select the input source in the monitor's menu under System > Input.