You cant. you can only go to johto and kanto.(if you have an arceus, you can also go to the sinjoh ruins, but that's sorta wierd and has only one purpose, plus its in northern johto anyway)
You take your Pokemon and delete all of them except for you starter Pokemon you defeat the elite four with it only, but first catch Luigua/Ho-oh and the 2 roaming dogs and a wobbufetet.