You have to have Grass to beat The first the you have to you lose after that
No its not an HM in leafgreen.
train your Pokemon that's it.
beat the elite four once
Beat the elite four.
go to One Island
You have to beat the Elite Four once.
nothing
all you have to do is not get beaten
beat the elite four and your starter has to be a bulbasaur
You have to beat the elite four and complete your pokedex.
You must beat the elite four first, then you can enter.
The same you can do in Fire Red.Read my answer to that here:What_can_you_do_after_you_beat_the_elite_four_in_pokemon_fire_red