Origin Form of Giratina

Go back to the place where you caught Giratina.

Zero_or_die's answer isn't correct. If you enter the Torn World from spear pillar, you'll find nothing. On route 214, there's an entrance to the forest on the east side of the route, that leads to the Spring Path. It will lead you to the Sendoff Spring, you have to go down to the entrance of Turnback Cave. You need to obtain the national pokedex in order to enter the cave. Inside the cave, you have to search for the room that has a portal to the torn world. Just keep searching and never turn back (lol) or else you'll return to the beginning. Once you've reached the room, use the portal. You will appear in a the Torn World, but in a different area, not visited before. There you can find the griceous orb, the item that changes giratina's form. Hope this helps! Please rate this answer!

It's in the cave where you used to catch girintina

You have to go to turnback cave and get to the end. Once you are there you pick up the item and go through the portal. Once there you will find the griserious orb. Give it to Giritina and h will transform.(You can only get there after you beat the Elite Four.)

You must first defeat the Pokemon League. Go to Turn Back Cave where you catch Giratina in Diamond an Pearl. Once you are at the end of Turn Back Cave. You will see a Portal that will lead you to the Distortion World. Go to where you caught Giratina. You will an item. When you pick it up. You will find the Griseous Orb. Once you are out (or in,) put the Griseous Orb on Giratina. And he will stay in his Origin Form.

NOTE : If you go in the Union Room or the Wi-Fi Club, he will go back in his other form. If you go in the Wi-Fi Plaza, he will stay in his Origin Form. Like I said. You must defeat the Pokemon League which will enable for you to do this.