Well, all my friends have Giritina's Origin Forme and they won't tell me how to get it they all say "We found it out by our selves and you need to too." So please tell me how to get it. :3)
After the Pokemon League, go to Turnback Cave, (where you came out after catching Giratina) and go through (it is random!). Go through to the Distortion World and get the special orb (forgot name), give it to Giratina and you get it's Origin Form.