I think by talking to dawn lil' sis'
I deleted the answer. In your faces.
Dialga
i know you can catch a ditto
luck
you need a action replay
go to the mansion garden
pee wee Herman
You can't catch Palkia in Pokemon Diamond, you'll have to trade with Pokemon Pearl
Go to serebii.net for all your Pokemon needs
aftewr you get your national pokedex you will get a place where you can catch it only of you have the bayleaf on Ruby.
No you can't you can catch it noly in pokemon dimond pearl or platinum
You can go to battle tower, vs you rival, catch legendary pokemon you can catch garratina in dimond look on youtube.