how do I hook up Isight web cam to 2013 mac mini?

Assuming you mean the 2012 Mac Mini since the only Mac Mini after the late 2012 Mac Mini is the current late 2014 Mac Mini you need a FireWire 800 male to Firewire 400 adopter. However It may not work well if at all.

I successfully used one with Mountain Lion on my 2012 Mini. With Mavericks it would only display one frame every 20 seconds or so. I have not tried it with Yosemite.

I did try using it on my 2009 Mac Pro with Yosemite and it interfered with Screen sharing so I gave up

Yes you have a 2014 Mini since the Min first shipped with Yosemite. Also, the 2014 does not have a FireWire port but you can get a Thunderbolt>FireWire adopter