How do I get deoxys,jirachi,celebi,mew and other legendary pokemon?

Zapdos- Catch in Power Plant.
Moltres- Catch on Mt. Ember.
Articuno- Catch at Seafoam Islands.
Mewtwo- Catch in Cerulean Cave/ Unknown dungeon.
Mew- Get through cheat system or Nintendo event.
Raikou- Catch in wild after beating Pokemon league if Starter Pokemon you chose was Squirtle or Trade from another Firered or Leafgreen.
Entei- Catch in wild after beating Pokemon league if Starter Pokemon you chose was Bulbasaur or Trade from another Firered or Leafgreen.
Suicune- Catch in wild after beating Pokemon league if Starter Pokemon you chose was Charmander or Trade from another Firered or Leafgreen.
Lugia- Catch on 8 island.
Ho-Oh- Catch on 8 island.
Celebi- Get from cheat system or from a Nintendo event.
Latias- Trade from Sapphire.
Latios- Trade from Ruby.
Regirock- Trade from Ruby or Sapphire.
Registeel- Trade from Ruby or Sapphire.
Regice- Trade from Ruby or Sapphire.
Rayquaza- Trade from Ruby or Sapphire.
Jirachi- Trade from Ruby or Sapphire.
Deoxys- Get from island 9 or from a Nintendo event.

You need to use a cheating device.

Hope I help

Deoxys: Birth Island
Jirachi: Pokemon Colosseum Bonus Disc
Celebi: Impossible to aquire without cheats
Mew: Rainbow Island in Emerald Version, then Trade it over
Entei: Appears randomly around Kanto when you pick Bulbasaur as the Starter
Raikou: Appears randomly around Kanto when you pick Squirtle as the Starter
Suicune: Appears randomly around Kanto when you pick Charmander as the Starter