In order to join a class, you need at least five talents (see ‘help talents’ to see how to unlock extra talent slots). You also need to do the following:
The black sash is earned in the dojo, and needed for all classes except dragon.
To class as a dragon you must have the Dragon Hatchling talent, as it unlocks the ‘travel’ command. After travelling to the Dragon Realm, you need to kill one of each of the four different dracamanders (each gives a different wisdom).
To class as a titan, simply type ‘find tartarus’ and follow the instructions.
To class as anything else, you should first fabricate a wand (and perhaps also a mount) from your home plane, and then travel to the island which is around 20,000 feet east of Glyphstone Village. The ‘area’ and ‘find’ commands should allow you to locate the appropriate dungeon, and the ‘area’ command can be used within each dungeon to help track down the boss. After killing the boss and earning one classing token, you will need to leave the dungeon to reset it.