Common ProblemsI can't find The Heart of Azeroth quest
Before obtaining this quest you must complete the quest chain for the Battle for Lordaeron, which starts with Hour of Reckoning. These quests start in Orgrimmar or Stormwind based on your faction.
After the Battle for Lordaeron, once you are level 50, if you are not pushed the quest, you will need to speak to the Earthen Guardian in Boralus (Alliance), or Dazar'alor (Horde), who will then send you to meet with Magni in Silithus, where the Heart of Azeroth quest begins.
If you don't get the Heart of Azeroth quest after completing those, please submit a bug report.