THIS CODE IS UNTESTED & THEREFORE MIGHT NOT WORK!!!!
THERE ARE 3 WAYS TO CATCH CELEBI:
1. First go to elm and talk to him, after that go to the nearest mart and get potion, heal a pokemon thats weak, but do not heal it at poke center, after go to helixforest and go to the shrine and there will stand an unusual pokemon, talk to it and it will start battling you and you'll have a chance to capture the 251 pokemon celebi.
2. When you start a new game press: A, A, A, B, B, B, B, Select, Start
3. Go to the ''Ruins of Alf'' and catch all 26 unowns in alphabetical order from a-z. When all of them are captured in order go to the man in the house at the ruins he will open a door for you. Go in and you''ll find Celebi the 251st pokemon.