use lots of max repels
you will go to route 2 peter city side
there is no diglet cave.
you get there by capturing snorlax
diglet is in diglets cave
go through diglets cave in vermillion city
wake up the snorlax at vermillion city, diglets cave and go through diglets cave. you should end up near pewter city so go down from that. PS to wake up snorlax you need pokegear upgrade and go to top of the round channel thingy and it should say poke flute then just talk to snorlax and it will wake up. it is level 50
You can get to the Viridian Forest in HeartGold by going through Diglett's Cave, an entrance to Diglett's Cave is in Vermilion City.
Nothing you could just find the Dugtrio or Diglett
Outside of diglets cave
you dont you battle mewtwo and get out
You buy a couple of repels (optional). Get through the cave. Sorry I don't know how
Ummm..... Diglett`s cave is not a secret, anyone who actually has played pokemon would know that...... And no you do not get torchic there.