You will be given your camera early on in the game, and told how to use it. Press
to open the camera, and
to take a photo. You will probably take photos in every chapter anyway, whether just admiring the scenery, doing tasks for other characters, or taking photos of white objects to collect papercraft models.
The one thing to watch out for is the short final chapter, The You, which it's all to easy to forget about. If you forget to take a photo here, then once you've completed the game then travel back to that bookmark and take a photo as soon as you can.