How can you change your hairstyle in Animal Crossing: Wild World?

In Animal Crossing: Wild World, Animal Crossing: City Folk, and Animal Crossing: New Leaf the player has the option to change their hairstyle at Harriet's barber shop, Shampoodle. In Wild World, Shampoodle is available inside of Nookington's, meaning that players will first have to unlock Nookington's by spending 240,000 bells and having a friend visit their town and purchase an item from Nookway before being able to change their hairstyle.

Source: Hairstyle - Nookipedia

Once in Shampoodle you will have several options available. A hair style guide can be found here, and a hair color guide can be located here

According to Animal Crossing wiki,

In Animal Crossing: Wild World, Animal Crossing: City Folk and Animal Crossing: New Leaf, the player can change their character's hairstyle by visiting Harriet at Shampoodle. The style and color is determined through a series of questions. When getting a haircut or makeover, Harriet will push a button, and a large, hairdryer looking device will cover the player's entire head. After a few seconds, it pops off. The player will always have a shocked look on their face when the action is complete.

A hair guide can be found here

If you have a friend over via WIFI or DS to DS and they buy something from your shop when it is Nook Way, then Tom Nook will upgrade. When it gets upgraded then in his store he will have Shampoodle in it.