If you're talking about Dialga, then you have to beat Cyrus at the team Galactic Veilstone City building. DO NOT leave without freeing Azelf, Mesprit and Uxie. Then go to the top of Mount Coronet (you will need Rock Climb, Surf, Strength and Rock Smash). When you get to the top of Mount Coronet (Spear Pillar), you will see Cyrus bringing Dialga to Spear Pillar from another dimension. Beat Cyrus and then go up to Dialga and press the A button. You will start a battle (listen to the music that plays during the battle as it is AWESOME!).
It is a good idea to save your game after beating Cyrus if you plan to catch Dialga, so if you accidentally defeat it you can reset the game.
cheats
you go to the spear pillar.
it would probably be shaymin because he's low level
Pokemon Diamond was released a tiny bit earlier as it is put first in bulbapedia lists and the creators needed time to change the lengendary from dialga to palkia.
You will need to catch the three lake lengendary pokemon. (Mesprit, Uxie, Azelf).
yes sort of
If you want to transfer a Pokemon to Pokemon Diamond and Pearl, you will need Nintendo Wi-Fi. You have to download the mission(s) and transfer to your DS Card.
No.
the lengendary birds are in caves ex. moltres is at mt. ember and the ground lengendary Pokemon pop out of the grass once in the game. it depends on what your starter Pokemon is to what lengendary ground Pokemon you'll get.
no, but it is awsome!!!
no they cant
Yes. You can. However, you must have traded it from another game. I have about three Rayquazas on my Pokemon Diamond because I have traded them there from all the Sapphire games I have restarted.