You can't catch Ho-Oh in Pokémon Platinum, you need to trade it from Pokémon Colosseum or with someone who has it.
you cant unless you have a game boy game that you catch it on and then migrate it to platinum
You can't get it without hacking or from an event
You cannot find Ho-Oh in Pokémon Platinum because he is not available in it, so you need to migrate it from the gameboy games.
You can't get it in-game, you can only get it through trades or through migration - that's it.