Once you get the HM cut go to viridian city and go north there will be some trees you can cut down if you go past it and go north you will find a big building in there will be a scientist who will give you HM flash only if you have 10 Pokemon recorded in your pokedex.
a city
other side of diglett tunnel
You need at least the first badge to use flash.
one of professor oaks aides gives it to you
get the item flash
in front of a cave not sure which 1 tho
it's either cut or flash i cant remember
use the move Flash (TM70)
cut,fly,surf,streangh,flash,rocksmash and water fall
You can use it in caves where you can't see very far and stuff like that.
You cannot. Flash Cannon was not introduced until Generation IV, and Fire Red is a Generation III game...and a Generation I game.
HM05 contains the move Flash which makes dark places light up.