If your World of Warcraft account is upgraded to the newest expansion, you can take advantage of the Level 48 Class Trial. You can use this feature to take a character for a test drive, see how a certain race or class looks in-game, or for any reason you may want to test out a new character. 

To start a new Class Trial, tick the Class Trial checkbox while creating a new character.

Your new trial character is playable for three hours, though, if you choose to play through Battle for Azeroth content, the class trial will end when you select a zone from the Scouting Map in Boralus/Dazar'alor even if it's been less than three hours. After the trial is over, your character is locked from further play. You can purchase and use a Character Boost to permanently unlock your trial character.

Note: You cannot enter raids or low-level dungeons while on a Class Trial, nor complete world quests.