You can change things in Display in the Control Panel.
You have to have something to put on the other monitor. You just direct the output of what you want to go there to that monitor (1 or 2). For example, if you want a slide show to show on it, you go to the slide show software and direct the output to it.
Or if you want half of your desktop to show on each monitor, make the screen resolution wide enough so that it will span the two monitors.