If the bands on the left and right aren't equally wide, equalize them with the horizontal position control. For our purposes, just big enough so you can see how black they are. But if the edges of your image look funny or different from the rest of the image, you might try making the image a little smaller, to get it away from the edges of the screen. These words will wait patiently for you. The icons for brightness and contrast are shown to the right.
Some monitors adjust even brightness and contrast on-screen.
If you don't know where your brightness and contrast controls are, look around and find them. Again, if all else fails, try to find the manual for your monitor and read it. Now compare the black rectangle to the right with the black bands around the screen. Adjust the brightness for the brightest setting that keeps the black rectangle as black as the black bands. If you can't quite do this, do the best you can.