When it comes to what software to run on your Raspberry Pi to get into Retro gaming the two most popular options at the moment are RetroPie and RecalBox.
Both of these are Linux based Operating Systems (OS) which when loaded onto an SD card and inserted into your Raspberry Pi will boot it up to the point where EmulationStation takes over. From there you are presented with a Graphical front-end showing your emulators and games that can be played.
While both Retropie and Recalbox use EmulationStation as their front-ends they approach it a little differently and offer a slightly different experience.
There are other options should you choose.
One is called PiPlay and came from the PiMAME project. It’s a little older and not quite as up to date as RetroPie or RecalBox but could still be worth a look.
The other is called Lakka, which is based on the emulator RetroArch. While the interface isn’t quite as pretty or user friendly as the other options it is still a great alternative.