Either you import them from previous games, use an Action Replay or wait for any official Pokemon events.
You don't
No you don't.
Nothing happens after that. You just caught the three legendary roaming Pokemon in the Johto Region.
You have to get a legendary Pokemon. so you can beat.
No it is a starter in the johto region.-evolved from a starter
Pokemon Pear [and Diamond] is located in the Sinnoh region. You will not be able to get to the Johto region unfortunately. Sorry.
You can't, only Sinnoh is available in Diamond.
afther you beat all the johto gym leders.
no
migrate them through palpark after you get the national pokedex.
You can't get to Johto region in Pokemon Diamond/ Pearl, but you start there in Heartgold/ Soulsilver.
I don't think you can.