You need to get an electric-type Pokemon and obtain TM 70 Flash, HM 6, HM4 and HM 3. You can obtain the TM 70 at the Sprout Tower in Violet City. Also, there will be tons of Geodude in the cave, so water and fighting types couldn't hurt either.
In Goldenrod City go to the department store (Poke-mart) 5th floor and buy the TM70(flash) and teach it to one of your Pokemon and use it while you are in a dark cave
you have to go through digglet's cave
walk?? look around!!
Check out this site it has maps of the Ice Cave http://www.pokemonelite2000.com/gsicepathmaps.html
The Silver Cave is located in Mt. Silver in which you can go after you get all the Johto and Kanto badges. In the Silver Cave you can find lots of strong Pokemon and fight the final boss, Red at the top of the cave.
You wake up Snorlax and go through Diglett's Cave.
It will appear in Mt. Silver cave.
You can't get into cerullean cave(the cave with mewtwo in it) on Pokemon silver because according to the game's story, the cave has caved in, but there may still be some remmants of the cave you could get to via a gameshark.
In Pokemon silver I think
Dark Cave
Silver cave in Pokemon Gold is located on route 28 on the way to Mt. Silver. The player will need Cut and Surf to navigate this passage.
Mewtwo is in cerulean cave, and you can only get Mew through an event.
Make sure you have the HM Cut, then go through Digglet's Cave.