RPI retro-gaming console with RetroPie
Power (and Internet for downloading games)
To transfer your RPI (or other supported SBCs) into a retro-gaming console you need a computer
with SD card reader and some software for flashing your SD card (I use Etcher
There are two ways of getting RetroPie to work. You can install it on top of existing OS or you can use pre-made image provided by RetroPie
I've used my Raspberry PI 2 and RetroPie's pre-made image.
Plug your SD card into your computer, open Etcher (or any other flashing software), select image and SD card and click Flash! button.
Then just wait until it finishes and unplug SD card from your computer.
Put that SD card into your Raspberry PI and setup your controller. And it's almost done, you also need some games to play.
RetroPie supports a lot of emulators and you can play games from consoles like NES, SNES and many other.