Aperture, shutter speed and ISO, the big 3 that usually cause the most confusion early on. If you can get a grasp on how the big 3 work together to affect your exposures, taking pictures will be a much more pleasant experience and will have a lot less frustration. Once you understand it, it is like riding a bike. It may take some time and effort to learn, but once you learn it you won't forget it. The one that causes the most confusion out of the big 3 is usually aperture. Why? Well there’s that sequence of numbers that seem to work backwards and that little matter of depth of field. Understand the big 3 and the rest will come much, much easier.