Once you reach Chapter two of the story, you'll be able to input passwords, allowing you to gain new items, among other things.

Password	Effect
5R642H8R	Apple x20
JQQPK0R6	Apple x20
8NSPF1W5	Oran Berry x20
JR6TW63Y	Reviver Seed x5
1XR446WJ	Reviver Seed x5
2943SK47	TM - Bulldoze
8WJNT45F	TM - Dragon Claw
95QTTSQT	TM - Power-Up Punch
T1W5R0PH	TM - Rock Slide
HCJXR8XN	TM - Swords Dance