Instructions
Travel to Saffron City and enter the Silph Company headquarters in the northern part of town.
Defeat Team Rocket in the Silph Company building. Giovanni, the Team Rocket leader, will give you the Silph Scope and a Masterball when you defeat him.
Travel to Lavender Town and enter the Lavander Tower. Walk to the second floor and engage in battle with a wild Pokemon. Use the Silph Scope to identify the Ghost Pokemon. You will be able to travel through the tower and attack the Ghost Pokemon from that point on.
Travel to the top of the tower and defeat the Marowak. Marowak is a Ground-type Pokemon, so use a Water-type or Grass-type to defeat it fairly easily. Go up to the next floor once Marowak is defeated.
Defeat the three Team Rocket grunts. Mr. Fuji will take you out of the tower and give you a Pokeflute after you defeat the grunts.