Tail Cave is the first Dungeon in The Legend of Zelda: Link’s Awakening. To reach it you need the sword, shield and the Tail Key, which you can get in the Mysterious Forest. The Tail Cave dungeon is located south of Mabe Village.
Tail Cave Map

Tail Cave Bosses
Rolling Bones

After you acquire Roc’s feather you can defeat Rolling Bones. Just jump over the spiked log and hit Rolling Bones with your sword.
Moldorm

Moldorm’s weakness is its glowing tail. Use your shield until Moldorm moves away and hit its glowing tail.
Secret Seashell

Return to the Tail Cave after you bought bombs from the shop in Mabe Village to get the secret seashell.
Tail Cave Video Guide
Link's Awakening (Switch Remake) - Tail Cave Guide