I suggest that you follow the route to the north. It will head through a Sandstorm and then turn West. Please note that you will need numerous HMs.
I Beleive that you beat the Elite 4, They are really the Elite 5 because of the champ.
you cant what where you thinking unless you have an action replay
Get a national Dex
Depends on what Game Card you are playing. In Diamond and Pearl, you can get in that big building that's in the East of the Resort Area. But to get in that building, you need to have a Pokemon with...
you get in the resort area by getting the national pokedex, and then the people blocking the area will be gone, hope this helped