The Mac mini comes with a small DVI to VGA adapter in the box. This adapter allows you to use your Mac mini's DVI port with a VGA-compatible display.

Replacement adapter
If you need to replace the DVI to VGA adapter that came with your Mac mini, this Apple DVI to VGA Display Adapter (model M8754G/A) is functionally equivalent, and can be purchased through the Apple Store. This is the same adapter that came with some other Macintosh computers.
Other adapters
You can also use Apple's DVI to Video Adapter (model M9267G/A), which outputs S-Video and Composite video, allowing you to connect your Mac mini directly to a television or older projector.