After clearing the first dungeon (and getting the grappling hook item), capture a fairy in a bottle (or buy a red potion) and head back to Outset Island (it is all the way on the bottom row of the overworld, the second square from the left). Visit your grandma in your house, target her and use the fairy/red potion bottle on her. She will give you some of her special, healing soup in return. When the soup is used up, get a free refill from your grandma anytime.