First you need to go the Kanto region, which can be accessed after beating the Elite 4.
Head to Pewter City.
Defeat Brock and you'll receive TM80 Rock Slide.
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not, you will need to use tm80, rock slide, on onix.
Rock Slide is not a TM in Leafgreen. In Leafgreen Rock Slide is a Move Tutor move and the Move Tutor is hidden somewhere in Rock Tunnel.
Rock Slide is TM80, which you get from defeating Brock. Unfortunately, that is the only one you can get. Consider Stone Edge from Battle Frontier as an alternative.
If you have a Pokemon that has a rock type move, then you can easily win. Rock slide works the best.
there's no moss rock in heartgold/soulsilver. To get it you need to trade from a different game. Sorry!