FFXIV Endwalker: How to Unlock the Sage

First and foremost, you need to own the Endwalker expansion. If you are a new player in Final Fantasy 14 you won't be able to play as a Sage right away. The new jobs require you to level any Disciple of War or Magic job to level 70 before you can pick up the quest to unlock it.  So just pick a class that you like and play it to level 70.  You can then swap over to Sage. 

If you meet those requirements, head to the Lower Decks in Limsa Lominsa and speak to the Sharlayan Maiden located at coordinates X:9.4 Y:12.9.