You go to cave near lilycove city and go until you find Boss of Magma team, and then you go to sootopolis city and you must go to cave of origin in sootopolis city, and you go until you find Groudon in Magma pool. Yeah, you must to take groudon to become your Pokemon, yeah yeah yeah!!!!
Can't get it anymore!
groudon is only in the hoen games so its not able to be caught in the sinno.
you have to unlock the dungeon called shimmer desert. it will be waiting for you at the end of the dungeon.
If it is in Pokemon ruby then you have to go past the volcano island while surfing and dive everywhere you can. eventually you will find a cave entrance, go in then dive up, go through the cave to get groudon. once you battle the aqua boss he will disappear and then you have to go to sootopolis to catch him.