I'm assuming you jumped down the ledge and are at Rustboro City. Is Mr. Briney at his cottage? If so, take his ship and go back to Slateport city, go to Mauville city, and go north from there until you reach Mt. Chimney.
If he's not there, go east of Rustboro to the Rustboro tunnel. There will be rocks you can break with Rock Smash if you haven't already done so, and once you go through the tunnel you will be at Verdanturf town. From there, keep going east until you reach Mauville, then go north to Mt. Chimney.
If you are still at Fallarbor town, you can simply backtrack to Mt. Chimney.
Once you are at Mt. Chimney, go up the cable car and fight Team Magma.
Source: I've played these games before!!!
Hope this helped!
