you just have to keep going and the legendary is garintina
girantina is in that cave at lv 70
yes beat the game then go to turn back cave next to veilstone city and get the legendary ? in the cave. find out what Pokemon it is when you go there. its giratina
yes, you can also get both dialga and palkia .(if you defeat them there in the wild) If you beat giratina, he'll be in turn back cave.
turn back cave
fullmoon island, mt. coronet, stark mt., turn back cave, snowpoint temple, lake valor, lake veriety, lake acuity
turn back cave
in turn back cave
you get pass turn back cave
turn back cave
turn back cave.
The turn back cave in Pokemon Platinum is in Seodoff Spring near veilstone city
after you got the national pokedex there will be a new lake cooled turn back cave near route 214.