After you defeat Blane, Bill will come up to you and talk about some stuff. Then he will take you to One Island. He will take you to The Pokemon Center there and Talk some more. Than tell you to go the Game Place or whatever on Two Island. When you get there the dude there will tell you to go to Three Island to go find his lost daughter, Lostelle. After you go to Three Island, you have to fight 4 Biker dudes. They all have poison type Pokemon levels 35-39. After that go north and take a left to get to Bond Bridge (I think) which will get you to the berry forest. In the berry forest Cut and Surf make it easier but is not required. Lostelle is at the end of the forest. She was scared by a lv 30 Hypno which you have to beat. Afterwards she takes you back to the Game Place and thanks you.
Also north of one Island you can catch Moltres Lv 50. It is really strong, so be careful. It is a long road but you can heal your Pokemon In a cave. At the end there is a maze that requires Strengh ( I can't spell) at the end Moltres is there.