http://bulbapedia.bulbagarden.net/wiki/Registeel_%28Pok%C3%A9mon%29
You can't its not a move in emerald.
The braille in Registeel's tomb on Poké Emerald says: "those who inherit our will, shine in the middle", which means use the HM move flash in the center of the the tomb.
In Registeel's cave, you read the inscription then head to the direct center of the room and use Flash.
Read the inscription and then go to the middle of the room and use Flash. The door will open and you can battle Registeel at the usual Lv. 40
Many like Bastiodon.
You obtain Flash Cannon after defeating the 7th Gym Leader, Byron in Canalave City.
Walk to the middle of the cave and use flash.
You cannot. Flash Cannon was not introduced until Generation IV, and Fire Red is a Generation III game...and a Generation I game.
You cannot get Flash Cannon in Sapphire, it was a new move made for Diamond, Pearl and above. It wasn't made when Sapphire, Ruby, Emerald, Leaf Green or Fire Red came out.
HM 05 is flash
No...You have to have the TM for any Pokemon to learn it...
You get it in the Granite Cave in Dewford Town.