In Pokemon Platinum, You Have To Go To Distortion World Which Is Available In The Storyline There, You Will Have a Wild Encounter With It.
you could get a giritina from a trade or action replay
no
You cannot catch it in Pokémon Plantinum, you can only trade it.
you need to beat or catch giritina.
After you catch giritina youll come to spring field and it will be there in the grass!
go to turn back cave
giritina, palkia and dialga
Im sorry but no
you have have to catch giritina at the spear pillar on mt coronet
trade for it, but kinda unlikely, or catch it if you are a gold member.
Transfer the Arceus from Platinum that was released in a special event.
You can't. Use Poke'transfer to get one from sinnoh.