Hardware is straightforward - because your Pioneer has a phono stage, you can plug your turntable into that and then use the pre-out from there into the audio in on the iMac.
As for software, Audacity is good, as is the built-in Garageband - but they'll record your albums in one long file - you might want to look at some others that will automatically split the albums up into individual files (though I can't immediately find one for Mac...)