Do you mean a second screen showing the same thing as on the laptop screen or a second screen showing something different to the laptop screen?
If you want the second screen to show the same thing as the laptop then this is easy.
Just plug the cable from the screen (VGA or HDMI) in to the laptop socket.
Then you normally have to hit a button on the keyboard to get it to display on the second screen. The button is often one of the "F" keys at the top.
It may be an "F" key with an icon of a small screen on it. Sometimes you have to press another key on the keyboard to get the F keys to work (it varies from laptop to laptop).
It sort of "wraps round" as you keep pressing the F key. As you press the specific F key there are normally three options - laptop screen only, laptop screen AND the second screen (both showing the same thing), or second screen only, then it goes back to laptop screen only and so on.