How can I get to the towns like Pallet Town and Viridian City and Pewter City on Pokemon Crystal?

You have to wake up the Snorlax in front of Digglet's cave by using the pokelullaby on the radio next to it. It will then wake up and attack you. After you defeat/capture it, go through the cave it was in front of and it will take you to the exit, right beside Viridian city. From there you can go to Pallet and the other towns you wish to visit.