All laptops come with a Function key and in combination with F1,2,3,4,5 will trigger a certain function such as sound up/down, brightness up/down, wireless on/off and external monitor on/off look for this on the keyboard and when the external monitor is connected press the function key in combination with the F key that has a screen or a square drawn on it.
The other way is to go into the desktop properties by right clicking on the desktop and hitting properties and then going to settings > advanced and choosing your monitors there.
But as epots said if you give us the model of your laptop and perhaps the video card (ati, nvidia, intel) we can help you out much better.
Regrads, Cyberdyne
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