Have All of Your Info Ready
Pre-ordering an iPhone Xs will require more than just your credit card number. You'll need to have some very specific information on hand.
At the very least you'll need to have your Apple ID and carrier login information ready to go when you pre-order. That includes the user name and password.
If you aren't the primary account holder, make sure you track down that information from your parents or significant other ahead of time.
You'll need your Social Security Number (or the account holder's Social Security Number), and the account billing information handy.
Some carriers might require you to ship the device to the primary billing address so if that's not your current shipping address, you'll need to address that right now.