Testing conducted by Apple in August 2017 using preproduction iPhone X units and software and accessory Apple USB-C Power Adapters (29W Model A1540, 61W Model A1718, 87W Model A1719). Fast-charge testing conducted with drained iPhone units. Charge time varies with environmental factors; actual results will vary.
It's likely that third party chargers will soon support the fast charging feature but for now you'll need one of Apple's USB-C chargers:
● Apple MNF82LL/A 87W USB-C Power Adapter - $79
● Apple MNF72LL/A 61W USB-C Power Adapter - $69
● Apple MJ262LL/A 29W USB-C Power Adapter - $49
In addition, you'll need a USB-C to Lightning cable:
● Apple MK0X2AM/A USB-C to Lightning Cable (1M) [Apple.com]- $25
Apple hasn't indicated if using the 87W charger will be quicker than using the 29W charger. Please follow iClarified on Twitter, Facebook, Google+, or RSS for updates.