Wireless Mini Keyboard with touch pad - while this keyboard isn't going to be the one you're going to write a novel on, it's great for light multimedia work. If you dedicate a Raspberry Pi to RetroPie, the most you'll ever likely to type is LOAD "*",8,1. Old computers do assume that keyboards are present, so having one handy will make your gaming go much more smoothly. This one can be use wired or wireless, and in either mode it just shows up as a USB keyboard and mouse.
(optional) Mini External USB Stereo Speaker - I mean, you don't have to have sound, but the bleeps, chiptunes and dreadful old speech synthesis is a huge part of the fun.
(optional) CPU heatsink - some emulators really hit your Raspberry Pi's processor hard, so some heat dissipation will help avoid thermal performance issues.
A monitor or TV to do the initial installation. Don't worry — once the TFT is set up, you don't need the external screen
A USB game controller - any of the Xbox-style Logitech controllers work well, but many people use NES look-alikes. The RetroPie setup guide assumes a Logitech controller, so it might be a bit more work to use a different one.
Some games. While we can't give direct links to games that have been abandoned, RetroPie does have some free titles and systems available. There are also modern retro-games, like Pondsoft's The Bear Essentials for emulated Commodore 64.
What you'll need to do:
Before you install the TFT screen, install RetroPie by following their excellent instructions. It's best to use a screen and keyboard for this.
Shut down the Raspberry Pi, and install the PiTFT. For the initial driver setup, you'll need to keep the external monitor attached, but once you've got the PiTFT working you won't need the monitor.
Restart RetroPie with your game controller attached — and play!