After beating all 8 Kanto Gym Leaders, visit Professor Oak and he'll open up the path to Silver Cave.
Go to Viridian and take the Route toward Victory Road. Now it will let you in where the passages were being blocked by two guys.
If you go up to where you would for the Elite Four. Your Rival will battle you and will battle you each time you try to go up there on Mondays or Wednesdays
Your Rival
Sneasel
Lv. 45
Crobat
Lv. 48
Magneton
Lv. 45
Gengar
Lv. 46
Alakazam
Lv. 46
Stage 2 Starter Lv. 50
Now head off to the Silver Cave. Make sure your Pokémon are around Lv. 70, the battle there will be tough
