get national pokedex. goto spring path.rockclimb.go right.go in cave.pass 3 pillers
In diamond and pearl, the above is correct. In platinum, follow cyrus into the Distortion World. There, solve the puzzles and then Giratina will battle you. After u catch/defeat it, Come back when u defeat the E4 to get the Griseous Orb, which changes Giratina's form.