Professor Oak's assitant will give it to you.
Oak's assistant WON'T give it to you. He does that in RBY/FR/LG. In Pokemon Gold, Silver, Crystal, HeartGold and SoulSilver, the Elder in Sprout Tower will give it to you. In GSC, Flash is a HM, in HG/SS it is a TM.
Chat with our AI personalities
Most electric types can learn flash like voltorb and magnemite.
You are given a TM when you will need it.
Flash is an initial move learned by Volbeat in Generation IV. For the Pokemon who can learn Flash through TM70 in Generation IV, see http://bulbapedia.bulbagarden.net/wiki/Flash_(move)#Generation_IV_2.
Bellsprout, abra, mareep those are all I know
In Pokemon Platinum, Flash is a TM.