In Turnback Cave after Spring Path on route 214 (must find in cave after many rooms)
You can find Giratina in the Distortion World.
you have to beat Cyrus in the distortion world
You cant , the distortion world is exclusive to platinum =-) .
While playing through the story you will end up in the distortion world with Cynthia. in the distortion world you will have to battle Cyrus after beating him Cynthia will heal your Pokemon. then you will be able to find giritina, in the distortion world.
go up and find the spear pillar
Giratina is not available in Leaf Green, he is catchable only in Platinum version.
Yes you're allowed to go to the distortion world again, but you have to go through Turnback cave to find it.
in sunyshore city after you have gone to spear pillair and distortion world
First you go to spear piller and then you go through a portal to the distortion world and that's where you'll find giratina the main platinum Pokemon.
After you beat Cyrus in a battle for the last time in Distortion world you will then try to catch Giratina.
the guy from the team galactic gives you one to catch girantina in the distortion world
go to sendoff spring and go left then go up until you find the portal then go in the portal and then you are there!