You cannot as rock smash was not introduced until Gold/Silver/Crystal. You can use HM 04 Strength to move certain boulders though.
Chat with our AI personalities
You can break rocks by using the move Rock Smash.
get the HM strength, and teach it to a Pokemon
There is no TM that can Break rocks in Pokemon leaf green there is however in HM that breaks rocks called RockSmash
HM06 Rock Smash
If you're talking about the small, rugged rocks, then I don't think you can. The only rocks I know you CAN move/break are the huge boulders made for Strength.