After you beat the hall of fame.So I can breed Pokemon
You need the National Pokedex. To do so, have the number of "caught" in the Pokedex at 60, and talk to Professor Oak.
In FireRed, talk to Celio in One Island's Pokemon Center. He will acknowledge that you have caught lots of Pokemon and say he's looking for 2 Gems. You need to give Celio both the Ruby and Sapphire.
The Ruby is located at Mt. Ember. When you start to ascend the mountain, you should see Rocket Grunts. Talk to them, beat them up, and go inside the cave they were blocking and get the Ruby. Give the Ruby to Celio and he will give you the Rainbow Pass to get to Four, Five, Six, and Seven Islands.
The Rainbow Pass will give you access to that Island.