Ho-Oh is not obtainable through normal gameplay in Emerald or LeafGreen. It requires external events or trading.
The Aurora Ticket (event item) unlocks Navel Rock (Ho-Oh/Lugia island) in Emerald/FireRed/LeafGreen. **No in-game way to get it.** Action Replay / Gameshark codes exist to force the ticket flag—but this is cheating, risks save corruption, and flags the Pokémon as "fateful encounter" missing (invalid for official tournaments).
Best Advice: Play Pokémon Colosseum/XD on GameCube/Wii (or Dolphin emulator) to earn Ho-Oh legitimately, then transfer forward. It's a fantastic RPG duo worth playing anyway.