You get it shortly after leaving the Shrine of Resurrection.
(Dead Zelda's Voice)
From out of nowhere, you hear a woman's voice speak to you... She instructs you to head to the marker displayed on the Sheikah Slate.
The Slate is both alien and somehow familiar to you... You find that you can access its map by pressing one of the buttons.
- Head towards the yellow marker on your mini-map
- Interact with the pedestal at that location.
From Hyrule Castle, off in the distance, you hear the woman's voice speaking to you again...
- This quest and the next few ones act as a sort of tutorial for the game
- There are various enemies to kill here, and a lot of loot to pick up, mostly some basic items such as mushrooms, starter weapons, and more.
- "Link... Link. Head for the point marked on the map in your Sheikah Slate."
- It is not clear exactly what triggers the mysterious voice to speak to you and start the quest; perhaps it is a certain amount of minutes played, or an area explored. Also it is not obvious whether or not it is mandatory to talk to the Old Man right after leaving the Shrine of Resurrection. More testing is needed to confirm.
Not mandatory, you can even go climb the cliff to your left first, fight bokoblins, visit temples, etc. then chat with the old guy later any time you want.