u must get the blu orb, then go to sootopolis city and go to cave of origin.
In Sootopolis City, at the side where the PokeMart is. Go up, and you'll find Cave of Origin
defeat team magma & aqua and go to sotopolis go to the gym then the cave
go and fight Wallace in the gym
To get past the man to enter the Cave of Origin you need to reach a certain point in the story. You will not miss the Cave of Origin as the story directly tells you go there once you've reached that point.
go to the cave of origin
go to the cave of origin in sootopolis
go to the cave of origin in sootopolis city
Sootopolis city but you cannot go in.
you do the event with kyogre and groudon battle then go to the left side of sootopolis and steven will be there then he will take you to the cave of origin. Wallace is in there then you go to the sky pillar where rayquaza is
Go too the underwater cavern and beat ether Archie or Maxie
You go in there after Kyogre is awakened, but until then, you don't.