In Moleville, you learn that a star crashed into the mountain and trapped some kids in the Moleville Mine.
Enter the Mine
When you first enter the Moleville Mine, there will be a cutscene. After you answer the questions, the moles will stand near the upper ledge. Jump onto the moles, then jump up to the ledge and go through the door.
Explore the Mine
Go east and there will be some Magmites that you can avoid or fight, and a Sparky that floats toward you. Keep going east to reach the next room.
You can optionally talk to the toad walking around on the ledge. Otherwise, go down below, then go through the northeast door.
If you go east, you find a spring. You have to jump on it to get through the dungeon, so jump on it. Croco will come and steal your coins. Afterward, go east and through the door, and follow the path to the end. There will be a cutscene. Go through the door that Croco opened, then follow the path until there is another cutscene.
Go through the southwest exit, then the southeast exit, then go east as Croco runs away. Before you go through the exit, you can optionally battle the Crook behind the crate to the north. After you defeat them, you get a Flower Tab. Go through the east exit to follow Croco.
There is a Crook hiding behind the spring, so walk up to it to battle it. You get another Flower Tab if you defeat them.
You can also go through the east exit and battle the Crook hiding behind the crates there for yet another Flower Tab.
At this point, Croco will just go in a loop through the room with the spring and the rooms connected to that. Just wait in one of the rooms, and press the attack button when Croco runs into you, and the battle with Croco in Moleville Mines will start.