How do you get the Pokemon Metagross in Pokemon Ruby?

You must defeat the Elite Four and Steven first. Once you've done this, fly to Mosdeep City, then go into the house towards the north-west of the city that has no people in it. There should be a Pokeball there and a note from Steven saying that he left the Pokemon inside for you, which is a level 5 Beldum. At level 20 it evolves into Metang, and then at level 45 Metang evolves into Metagross.

After beating the Elite Four, head to Steven's house in Mossdeep. In there, take the pokeball. The Pokeball will give you Beldum, which you will have to level up to Metagross