After you defeat the elite four and champion (and complete the regional pokedex) you will be allowed to take a cruise to Kanto. In Kanto, the entrance to Diglett's cave is blocked off by a sleeping Snorlax. Later, after you have gotten the upgrade to your radio in Lavender Town, you can return to this Snorlax. Go to your radio and manually tune it to the upmost point in the circle. You will be tuned into a program called "pokeflute" once you have done this, press A when adjacent and facing the sleeping Snorlax and he will wake up and battle you. After you are done with the battle, Diglett's Cave will be right nearby as an enterance to a cave on one of your sides.
Petwer city
you need to go to diglett cave.
it will be in front of the diglett cave,,,,,,,,,,in kanto in vermillion city.........
use pokeflut
Snorlax is found blocking diglett's cave in vermilion city.
You have to wake up the Snorlax in front of Diglett's cave and catch him. He is holding leftovers.
diglett cave you have to wake the snorlax first with the poke flute radio channel
after you wake up snorlax u can go through diglett cave and get to pewter city
you have to get the Pokemon flute from an old man or team rockets headquarters
After defeating the champion in heartgold and going through diglett's cave
Here is a list of Pokemon Cities or Towns you must beat IN KANTO in order to reach Diglett's Cave:Vermillion CitySaffron City (OPTIONAL - RECOMMENDED)Cerulean CityLavender TownOnce you have passed these four places, you should be able to reach Diglett's Cave.(The whole process to get through Diglett's Cave is a Side Quest. These places are the places you will need to go to in order to complete the Side Quest.)
Go to diglett cave.