To push away rocks, you need to utilize the move Strength
You can find Strenght by doing the following:
Before you can get strenght, you need to have Rock Smash which you can get from a guy in Mauville City(in a house)
Secondly go to Rustburo or Verdanturf
Enter the Rusturf tunnel and there should be a karate guy standing near breakable rocks.
Then use Rock Smash on the rocks. When you do this the man will be so happy on the way out of the tunnel he will give you the HM as thanks for letting him go see his girlfriend.