How do I use an external monitor?

[Tackie]

I use laptop, and want to use an external monitor which is connected, but it doesn't work.  How do I switch to external monitor from built-in monitor?
I checked Settings but I found no switch-page.

[bin]

Normally Settings>Display will show you both monitors and you choose the configuration from there.

If the external monitor is not showing it is either broken, or not being seen by the laptop.
So, what connection is it? HDMI, Display Port, VGA, DVI-D?
What make is the monitor - Dells have their own quirks.

Next, what graphics card?

Inxi -F should help provide more assistance. Copy and paste the output.