In the properties of the light, if the light is a spot light, under the Effects tab is a setting for 'Light Cone' Click on the 'Edit' button, play with the settings to your liking, and render. :D There is a small render window in the dialog box for Light Cone that allows for making adjustments. Since Light cone is a post-render effect, the image just needs to be rendered once, and then the settings can be played with showing quick changes as needed. Have fun!