how to get celebi in pokemon crystal gbc

You get Celebi in Pokémon Crystal by using the GS Ball event and then taking it to the shrine in Ilex Forest. In the original Game Boy Color version, the normal way was tied to the Mobile System event, while later releases and some setups can unlock the same event through other means.

Standard method

1. Obtain the GS Ball.
2. Take it to Kurt in Azalea Town and wait until he gives it back.
3. Go to Ilex Forest.
4. Use Cut if needed to reach the shrine.
5. Interact with the shrine and place the GS Ball there.
6. Celebi will appear as a battle, and you can catch it.

Important note

For the original Crystal on GBC, the GS Ball was originally linked to the Japanese Mobile System event, so many players outside that setup could not get it normally. In the Virtual Console version, the GS Ball event is available after beating the Elite Four and you receive it in Goldenrod City.

Quick caveat

If you are asking about unmodified original GBC Crystal , the legitimate in-game route is the GS Ball event; otherwise people usually rely on glitches, cheats, or event-unlock methods.

TL;DR: Get the GS Ball, have Kurt inspect it, then put it in the Ilex Forest shrine to trigger Celebi.