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.
You can just check the dressing room (curtains) at any pokemon center or (I think) the mirror in your house. There's a menu of clothes to try. It's a simple vertical list of all clothes you own in a certain category, just select items to switch, no complicated controls. You can remove "accessory" items (worn on your hat) or socks/stockings by re-selecting the currently worn item, but most other items can't be removed (not even hats).
However you might need some different clothes to change into, I don't recall starting the game with any options. Towns you find with clothes will have "boutique" listed by them on the town map (george has a list). The first boutique is in the third town, right after the first forest, though it only has a selection of hats.