To enable a monitor, extend the desktop to it.
The position of the monitors in Display Properties controls how the mouse moves between monitors. In the screenshot to the right, monitor 1 is to the left of monitor 2. This means that if the mouse is on monitor 1, and you move the mouse to the right, it will move onto monitor 2. For natural mouse movement, the position of the monitors in Display Properties should match the position of the actual monitors on your desk.
To change the position of a monitor, drag its icon with the mouse to the desired position. Monitors can be arranged horizontally or stacked vertically, the only requirement is that each monitor touch at least one other monitor in a single point. To see which monitor has been assigned which number, click on Identify.
You can also select your main or primary monitor, this is the monitor which will contain the taskbar and start menu, and which will display the login screen.
Click on Advanced to change settings such as the refresh rate, color depth or monitor profile.