The result if your screen is not calibrated is that when you'll print your photos, you'll likely get prints that will look darker and more contrasted (similar to the calibrated version) than what your screen displays (non calibrated).
One can calibrate a screen by using free softwares that can be found online however calibration in this case is done manually and is not accurate. Much better, you can buy or rent a calibration device which will automatically adjust your screen luminosity and colour accurately through a scan.
This way your screen will accurately reflect colours and luminosity and your prints will be closer to what you visualised on screen and had in your mind.
Author : Rev'elle Photo, Lyon based photographer (France), http://www.revellephoto.fr