U have to go to the CLADON mansion and talk to the lady on the first floor with all the Pokemon around her and she gives u some tea u give to the guard.
The mansion is the building on the right of the big shopping centre
punani
you have to get through silph co and beat giovanni again to get the guard to go away!!! i know it sucks the people who made it that way are M-E-A-N mean
You need to have a national dex and need to have caught 70 or 110 pokemon.(I don't remember the exact number and whether you have to see or catch these number of pokemons but I am sure you have to do all these to get past the cave.You will get a Mewtwo in there).
you need to get tea in celedon city and give it to the guard
you use the smoke bombs to make the guard blinded for a few seconds then quickly get past him.
At the top of Celdon city , well there is a building there and right at the last floor you can go to some vending machines and buy water and give it to them
If your playing the original red version you need to just hand the water to the guard, if your playing firered you need to get Tea from an old lady in the celadon mansion and give that to the guard.
Celadon Pokemart -> 5th floor -> rooftop -> vending machines
go to lavender town see ya chumps
After you beat red
Any Pokemon that existed in Red, Blue, or Yellow.
after beating the members of the nuget bridge on route 24