detecting in your world that you are at a place to start this dialog. This is identical to any “pickup item” or “interact object” or “talk to NPC” or “get NPC mission” system
some kind of UI design, perhaps a dialog box, pick list, button list, etc.
That should give you plenty to start googling.
There are literally millions of tutorials for both of these things so get started today.
Start simple, just display something when you reach the spot.
When that works, instantiate a dialog and halt gameplay when you get near it.
etc. etc.
I suggest you take it one step at a time, just like this guy: