Now that you've got your Radio Upgrade from Lavender Town's Radio Tower go to your Pokegear and go to your Radio slot. Push up to tune it to the Pokeflute station.

Then exit the Radio and press A against Snorlax. You better catch this one, he can make a good Team Member.

Attached to Snorlax is an item called Leftovers. Leftovers will increase your Pokemon's HP after each attack turn is over.

Now you can enter Diglett's Cave since Snorlax is not blocking the cave anymore. Head inside, fight some Diglett or Dugtrio, then exit the other side heading for Pewter City.

Once outside, visit the fat guy in the little house south of the exit. He'll give you a Nugget for talking to him.