All macs that have a built in display also have a ‘video out’ port. It will look like one of these:
An external VGA Monitor which are the cheapest and most common external monitors you can buy, have a lead like this:
So to plug them in you need an apple display adapter that looks like this:
A note of warning. There are two different kind of display adapters, because there are two kinds of mac ports. Make sure you tell the apple guy when you order what model mac you have (eg macbook, imac etc) so that you get the right adapter.
Plug it in, and your mac will automatically find it and you’ll have a second display!