Like every previous game in the Pokemon series Pokemon Platinum hosts a variety of legendary pokemon for you to seek out. Finding all Pokemon on Pokemon Platinum takes a little luck and some strategy, simply because some of them like to wander the land rather than wait to be caught.
Giratina is one of the most sought after Pokemon in the Platinum version and it can only be found within the distortion world of Pokemon. It can be found at the end of the distortion world. Battling Giratina is a part of the main storyline of Platinum so you will have to either defeat or catch it in order for the story to progress. When you leave the distortion world Giratina will revert to its alternate form.
Uxie, Mesprit and Azelf
The legendary trio of Uxie, Mesprit and Azelf will become scattered across the map once you have completed your battle against Giratina and have spoken to Professor Rowan. Uxie will now be at lake Acuity, Mespuxie mesprit Azelf will be at lake Valor and Azelf at lake Verity. All three are at level 50 and Mesprit will flee and wander around the map once you initially approach it so tracking will be involved if you really want to catch it.
Dialga and Palkia
Dialga and Palkia can be found at the top of Spear Pillar once you have obtained the Adamant and Lustrous Orbs from Mount Cornet. When you have the orbs speak to the woman inside the large house in Celeste town. Make your way to Spear Pillar to find a bluish portal to Dialg
a, battle Dialga and exit the Pillar. When you return you should be faced with a now pink portal to Palkia.
Cresselia is a wandering legendary Pokemon, so once you approach it first it will flee and wander the map.Like any other wanderer it will take a balance of luck and strategy to find and capture it. Cresselia can be found during your quest for the Lunar Wing to heal the sailor's son. The quest will take you to Fullmoon island, Cresselia will fly away leaving you the Lunar Wing.
When you have completed the quest at Stark Mountain return to the Mountain, to the area where Charon was arrested and Heatran will appear. It is level 50 which shouldn't be a problem at this stage in the game.
Articuno, Zapdos and Moltres
If you have played the earlier versions of Pokemon that originally included these three legendaries you will probably already have a strategy for catching wanderers. When you get the national pokedex and speak to Professor Oak in Eterna city he will inform you that the trio is wandering Sinnoh and you will then be able to track them, or hope to stumble across them. All three are at level 60.