How do I get Garatina in Pokemon Pearl?

First, complete the pokedex. Then, continue to play the game for a few days. Then, check your town map. A certain pace will be by Vielstone City that wasn't there before. This place is the spring path. Bring a lot of repels with you, because there is a lot of tall grass. Continue through until you reach Turnback Cave. You need to find your way through the cave, and go through 3 rooms with 3 pillars. To make sure you are progressing, go up and click on the pillars. It will say 1, 2, or 3. After the third pillar, go straight. You know what to do.