After you beat the 4th, grass, gym you can go to the middle house in the bottom right of the city and grab a vulpix from the lady in there. If you check the Vulpix, it has HM 04, strength, as a held item
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HM STRENGTH HM STRENGTH
you get hm strength and hm defog after defeating two gym leader.Fantina(defog) Byron(strength).
get the HM strength, and teach it to a Pokemon
You get the hm strength and give it to one of your pokemon
Veilstone.