Gather Your Important Info Now
You'll need a decent amount of information to buy the Galaxy S8. This is not just your account login and your credit card.
If you aren't the primary account holder, you may need the last four digits of their social security number. You may need an account password and the other billing information like the address and zip code.
When ordering from a carrier, you will likely need to ship the phone directly to the primary billing address. If you are on a family plan and are at college or simply live across the country this could be a major issue.
You'll need this information if you go into a store and you may need the primary account holder to authorize you if they haven't already done so.