Go back to New Bark Town and talk to professor elm. he will give you then master ball. then fly to ecruteak, go to the kimono house and beat all 5 kimono girls (You will then get the Tidal Bell). a girl will then tell that she spotted lugia over whirl islands. fly there, and get to lugia, and capture or kill it. (you have a master ball, but if you don't want to use it yet that's fine). then fly to New Bark Town again, and go to the body of water on the right, and surf to the next route. go through tohjo falls, and across the route. Eventually you'll get to victory road. go through, and you're at the indigo plateau.