Go to the Game Corner in Celadon City and fight the team rocket grunt that is standing near the poster. Fight him and talk to the poster after he leaves. Press the hidden switch under the poster the grunt was garding and go down the stairs when they appear. After you have been through the Rockets Hideout and beaten Gaovianni, he will drop the Silph Scope.
Next, go to the Pokemon Tower in Lavender Town and go to the place where the last set of stairs are. Walk to it and battle the ghost. You will battle the ghost of Cubone's mother. It is Marowak (at level 30). Once you have beaten it, proceed up the stairs.
Battle all the upcoming rockets and save the old man. He will give you the PokeFlute. Use this to wake the Snorlax.
Hope I helped!