The SD card in a new 3DS is behind the back cover, so you'll need to unscrew it to get at it. Also, the card is a micro SD card, so to plug it into your computer you'll probably need an Micro SD -> SD card adaptor, as most computers just have a standard SD card input. When you get it in your computer there should be a folder called DCIM which is where all your photos are saved.
If you don't have an adaptor or a computer with an SD input, this method would be better.