You can get Dialga, Palkia, and Giratina in Heartgold and Soulsilver at Lv. 1. You need to have an event Arceus, and go to the Ruins of Alph, and then you'll be teleported by some Unown to this place called Sinjo Ruins. And then after you deposit all your Pokemon besides Arceus in this PC up there, you go to this trianglur stage, and Arceus makes this blue/pink/brown shape egg thing, which hatches into Dialga/Palkia/Giratina, depending on which one you chose.
Sinjoh Ruins - Bulbapedia, the community-driven Pokémon encyclopedia
I'll keep it. I might train it but I don't know. I hope the next Telepathy Dialga I get is better. I just hope that it's Sp. Atk is a bit better. I think 20 Sp. Atk IV would be nice. And also, I hope for a Modest nature, but the next best nature would be Quiet, and then the next best after Quiet would be all the neutral natures, like Serious. Wish me luck for the next Dialga... Because I won't get another chance after that.