"The Legend of Zelda: The Phantom Hourglass" is the spiritual sequel to the GameCube game "The Legend of Zelda: The Wind Waker." In the game, you play as Link, the Hero of Time, as you traverse the sunken world of Hyrule on yet another quest to save the kingdom. If you ever beat the game and want to relive the adventure from the very beginning or just got the game from a friend and want to start your own quest, resetting your game is easy.
Turn on the Nintendo DS with the game cartridge inserted and tap the warning screen with the stylus to bring up the main DS menu.
Tap the "The Legend of Zelda: The Phantom Hourglass" icon with the stylus to bring up the game's main menu, and then tap the screen again to bring up the File Selection screen.
Select the game file you want to reset by tapping its name with the stylus, which will bring up the File Selection menu. There should be two game files; they will be named whatever you or your friend specified previously.
Tap "Erase" at the bottom of the screen, and then tap "Yes" when asked if you're sure. This will erase all of the game data, labelling the game file as "New Game" when you try to access it again, allowing you to start the game from the very beginning.
If you're ever planning on trading in your game to a used game retailer, you should always reset all of the game data.