After you defeat Bowyer in the Forest Maze, you can go west to the town of Moleville.

Visit the Item Shop

Be sure to check out the item shop. Be careful when buying armor, because you can only wear one armor at a time, but the shop sells multiple types: "Mega" armor that is specific to Mario, Mallow, and Geno, as well as Work Pants, which all three can equip. The item shop also sells better weapons, and better mushrooms and syrups that restore more HP and FP during battle.

Explore Town

Go to the northeast part of town and there will be a cutscene.

Then go to the mine entrance just west of where you saw the cutscene, and talk to the mole. There will be a brief cutscene.

You can optionally go to the top of the mountain and talk to the mole there.

When you're ready to continue, go into the mine entrance to reach the Moleville Coal Mines.