You have to go after the galactic grunt who blew up the great marsh.You will battle him near lake valor. Then cynthia will come and give you secret potion by which you will be able to go to celestic town.
Chat with our AI personalities
go to the Pokemon league and defeat them
Go to pastoria and battle the gym leader wake
go to end of distortion world and talk to him
go back to jubilife city then to get to canalave city use surf at the left of jubilife city.Or go to hearthome city and try to deafeat fantina.Or if in platinum go to pastoria city and battle Wake.
you fight it and defeat it w/ ur Pokemon