What do you do when you get to lavender town?
go to celadon city and get the silph scope in rocket hideout behind poster at games corner
In Lavender town, first you need the scope to see the ghosts, you get that from the tower in the town to the left of Lavender. Beat the Rockets and then return to lavender with the scope, use it when you're in the tower with Ghosts, beat marrowack and then the Rockets inside. Mr fuji (?) will give you the flute.