how do you get legendary pokemon

Well in Sapphire you will see Kyogre no matter what. You can also get The regis and Rayquasa. Don't forget latios and latias. Some you will get during the game. To get them you have to catch them, weaken them and give it a shot.

Cathc them by progressing in the storyline. Duh!

Unlock the Regi Caves by going to the under water cave by Paciflidog and use dig at the far back wall. Go to Mirage island and go up to the top of the tower to get Rayquaza. After Defeating the Elite Four and the Champion you go to your house and here the reports of the mysterious red creature flying over Hoenn. You can then use your map to track down Latias (the only one available in the wild on Sapphire) she is in a different location every time you enter a building. So try to avoid buildings while on the search for Latias. I also suggest that you save your master ball and have a Pokemon that knows Mean Look because Latias flees easily. I hope that I have helped you in this quest.