First thing to do is to create a new pic image called matrix. The next thing you want to do is to change the width and the height of your picture, as well as the resolution. Pick any background color you want, take the paint bucket tool and fill the background. To get the matrix code, go to Filter, Texture, and then finally Grain. You want to set the intensity to 90, contrast to 50, and grain type should be vertical. To get the grain to be green, you press Ctrl+U, press colorize and drag the arrow to the green you want. Finally, customize it to the way you want and you have a Matrix background.