[Update: The process is now live! You can find full instructions here.]
Later this month on August 27, the floodgates will be opened and World of Warcraft Classic will finally arrive. Given that folks are understandably invested in this massive time sink of a game, Blizzard is allowing for certain pre-launch concessions, most notably the ability to reserve your name ahead of time.
Starting today at 6PM ET, Blizzard will let folks pre-select realms (servers) and reserve your names so that you can coordinate with friends and play together. To do so you need to active subscription on your account (reminder: WoW Classic shares the same subscription fee as the base game), and you can reserve up to three characters today. Here is a list of servers to help plan your attack.
Blizzard also reminds us that you can only play with a single faction (Horde or Alliance) per PVP server as Classic is following the old realm rules, and there is a maximum of 10 characters per realm, with a max of 50 characters per region. Thankfully there's no overlap with the main game, as the 50 character thing is a separate entity and only applies to Classic.
Provided that your name is something basic like "Greg," be prepared to jump into the breach later today! As a reminder the classic character classes are: Warrior, Druid, Hunter, Mage, Shaman (Horde only), Paladin (Alliance only), Priest, Rogue, and Warlock.
Reserve your name [Battle.net]