If you mean to catch lugia, then it is the tidal bell. To get the tidal bell, follow these instructions.
1. Beat all 8 gyms.
2. Go to Prof.Oak.
3. He will tell you the Kimono girls want you.
4. Go to the Kimono girls.
5. Battle and defeat them.
6. After that they will give you the tidal bell.
Good luck!
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To catch Ho-oh you need a clear bell and the wing. For Lugia it's the silver wing and a tidel bell.
you need the silver wing
You don't 'use' it. You need silver wing to get Lugia. It's located in one of the four islands to the south without Silver Wing in your pack it won't apear. Same with Ho-Oh except with the Rainbow Wing.
The Silver Wing is what you need to be able to see Lugia. In between Olivine City and Cianwood City, there are the Whirl Islands. There are four entrances (you need HM07 Whirlpool to reach the islands) but only one of four entrances will lead you to Lugia. Other than merely having the Silver Wind in your possession/bag, you don't need to do anything else with it.
You need to go to Pewter City. Near the Pokemart, there is a man standing on a ledge and he gives you the Silver Wing needed to get to Lugia.