You must have the 5th or 4th gym badge go to mt mortar cave and as soon as you go to the entrance you see a hiker bump into you! So he says sorry little boy don't cry and give you the move strength! Happy I could he :)
How Do You Get The Hm Strength In Pokemon Heart Gold?
Go to mortar cave and man comes out and gives you strength
It's in the house next to the pokemon center in olivine city
Open the box nd it goes straight in to the wall
Man go bang and hit you