Spare USB C to Lighting Cable
The iPhone Xs comes with a USB to Lightning cable, but that's not the only one you will need.
If you want to fast charge you will need to pick up a USB C to Lightning Cable and a fast charger. The best case scenario is to get two to three fast chargers if you are a mobile user, but even just a second cable will help out.
I keep one in my backpack to connect to a USB C battery pack and to connect to my MacBook Pro. I also keep one plugged into a fast charger at home so I can top off quickly. The third rotates in and out of cars or as a spare when someone needs to borrow one.
Right now you can only buy official USB C to Lightning cords from Apple. We will see certified options possibly later this year or in 2019. Be very cautious of any unknown third party cables as they can damage your phone easier than cheap USB to Lightning cables you might be used to.