Software first. If you have GarageBand on the Mac, that would work - or audacity, which is a free download.
Easiest and perhaps simplest route - download Audacity. Connect your iMac's inputs to the old mixer you have. job done.
Of course, the snags really just start then. You need at the very least headphones. If you have separate inputs and outputs on the Mac, then whatever plugged into the mixer will work - it will take a bit of fiddling to get the levels right.
Easiest better quality route would be a cheap interface - look up focusrite Scarlett 2i2. It can have mics or line level inputs. It has headphones and other outputs for a speaker system/amp. Maybe Reaper or Cubase elements as a DAW - more useful with effects and instruments too.
This will either make sense to be gibberish. If it's confusing then you need to read around - including posts on this forum and maybe look at Sound on Sound Magazine - things like that to get the basics into your head.