Pokémon Diamond and Pearl/Route 204 & Ravaged Path

Route 204
After returning to Jubilife after getting your first Gym Badge, head north to Route 204. This route contains many items and wild Pokémon.

Ravaged Path
From here, all you need to do is go to the right and use Rock Smash on one of the rocks and exit through the newly accesible doorway. You may also go to the left and smash some more rocks and collect an item (TM 39 Rock Tomb).

Route 204
From your position after leaving the Ravaged Path, go a couple steps west and then north on to Floamora Town.