In this post I'm going to give a quick overview as well as provide some resources for downloading song packs and whatnot, as it's a little bit unclear how to get started with this game.
So, for those of you who have fond memories of playing DDR at your local arcade, great news! You can play it at home basically for free! It's a shit load of fun, and doubles as an excellent cardio workout that you can do in your living room (if you don't live above people in a wood apartment building, that is).
If you have kids, it's also fun times for fun families (just make sure to do a cursory review of the songs and remove any that have any drug/sex references in them, they're rare but they pop up from time to time). Get the energy out of the kids & help build and improve their rhythm.
The software works on all major operating systems (including Linux) although from what I've read support for all versions of OSX is a bit dicey. You can download it from the official website here.
Out of the box it only comes with three songs. The main repository for downloading new song packs is here:
https://search.stepmaniaonline.net/
All of the old/original arcade song packs are also available for download on this website that looks like it was designed in 2003:
https://zenius-i-vanisher.com/v5.2/simfiles.php?category=simfiles
Furthermore, there's a good beginner FAQ here.
On Windows once you download the song packs they can be installed here:
C:\Games\Stepmania\Songs\StepMania 5\Songs
Not sure about Linux/Mac but I'm sure it's equally straightforward.
The dance pads themselves you can just buy online and they cost about $30. There are more advanced ones as well of course but if you're just getting started the basic ones work fine.
Additionally there is a decent 
community for the game.
PSA: People also play this game on their keyboards and there are song packs specifically designed for those players. Don't make the same mistake I did and try to play them with your feet they're absolutely impossibly difficult even on the easiest setting. Make sure you're downloading the right type of simpack!
PSA2: Different song packs are of course programmed by different people who have different ideas of what "easy" is. The standard DDR songpacks from the original arcade games are all very straightforward, but a lot of the other custom ones are way harder even on the easy settings, so just a heads up there.
I think that covers all the basics. Happy dancing!
