How do I change clothes?

I'm trying to figure out how to change my clothes - I read that it is in front of the mirror - but cant figure out what buttons to press? Do I use the stylus?

There are two seperate parts to changing clothes: The first time at your house, and out on your adventure.

At your house after you wake at the start of the game, stand in front of the mirror and press "A". This will change your clothes.

Later in the game, you can find clothing stores in various towns. You can change your outfit in the dressing room at the top right of the store, or in a Pokemon Center to the top left. In either case, simply interact with/enter the dressing room and you will get the change clothes menu.

In order to have the different outfits and items available, simply buy them at the stores by interacting with them. Note that only the items at the back, in the middle area of the wall, are for sale, and these change on a regular basis.