You have to go to hearthome city and talk to fantina and she vil give you celebi
You get it by catching all the unknown including the 2 special ones you get after catching all the unknown in solaceon town then I believe you have to translate the writing then follow what it Say then walk for like some time and done but I really doubt this works I Just heard bout it and sounded realistic so yeah might not work sorry :* also I hear you can do this also for mew but I am pretty sure it won't work lol sorry
Wait for a celebi event or you can trade.. I have one and can give you one for an Arceus. .