its very easy :)
hafter beating Pokemon league go to slateport city and head at route 103.
go to that lake at this route and what is that?IT'S A NEW CAVE!!!!
and the cave its named ALTERING CAVE
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It is Altering Cave which you can enter only after you beat the Pokemon league.
There is no altering cave in pookemon red since the sevii islands don't exist in Pokemon red only in Pokemon firered and leafgreen in six island after going past the forest and the altering cave is also in Pokemon emerald in route 103 a cave appears after beating the elite 4.
go north from littleroot town .. just keep going
First, you go to oldale town.Then, go straight to route 103 and last there is May
You can't, you have to catch him in 102, there are none in 103, at least not that I know of, check your Pokedex.