The Mac Mini does not come with a display (or a keyboard or a mouse).
But the good news is that most of the computer displays in your local Best Buy or Walmart will work just fine with your Mac Mini.
You can use an “older” monitor (i.e. made more than five years ago) with your Mac Mini but it may need a special adapter cable:

Such cables are readily available and not terribly expensive but if you have an older computer monitor that looks something like this:

I’d have to recommend purchasing a new flat panel LCD:

NOTE: “LCD” stands for liquid crystal display.
Compatibility with your Mac Mini will more than likely not be an issue.
In addition to taking up a lot less space on your desk, these displays are easier on the eyes and use less energy than the bulky computer monitors of the past.
They’ve also come way down in price lately!
At the time of this writing, you can buy a 22″ Acer Widescreen TFT LCD Monitor on Amazon for $164.99:

I like watching movies on my computer (plus I prefer a little more screen real estate) so I opted for a 24″ model.
Apple’s 24-inch LED Cinema display is certainly not the cheapest visual interface for your Mac Mini but it is the very best display your stalwart & intrepid author has ever laid eyes on.

In addition to its stunning appearance, this display has built-in speakers and a microphone which he needs to create his computer help DVDs.
(You can also use the Apple display with a MacBook.)