You have to go to lavander town and have your Radio card upgraded. (To do this, get the machine part from the Cerulean City gym and return it to the power plant, then talk to the director of the Lavender Radio Tower.) Once you have the upgrade, go in front of the sleeping snorlax and choose the pokeflute song. Once you do that the battle with Snorlax will begin. His moveset is as follows:
Level 50, Item: Leftovers
I recommend Ultra Balls or Heavy Balls if you can get them. If you accidentally knock him out, just rematch the elite 4 and he'll come back.
After you battle him, you can go through the Diglett's cave and head out to Pewter City.