How do I connect a dell monitor to my i mac for dual monitors

The brand doesn't matter. Just plug it into whatever port works. You might need an appropriate adapter, such as an HDMI to VGA or a Thunderbolt to DVI; it depends on which Mini you have and what input ports are on your monitor.

When using an external display with an iMac, it's called video mirroring - take a look at this Apple article:

http://support.apple.com/kb/HT5019