I like to point people to http://www.cambridgeincolour.com/
to learn their way around most of the concepts of photography.
The hardest thing but most valuable thing to learn is using focal length for perspective instead of just framing. It's not a natural concept, but one you realize how to use it, it adds a whole new dimension to your photography. It's simple in theory - wide angles make everything look further apart and telephotos compress. Unfortunately it's hard to actually visualize it. The thing that helps most is looking at a subject and deciding how big or small you want objects in the background to be. For example, if you are shooting a building with a fountain and want the fountain to be big with the building off in the distance, use a wide angle (like a 24mm) lens and get close to the fountain. On the other hand, if you want the building to look bigger (maybe towering over the fountain), then you'd step back and switch to a telephoto lens (like a 200m) to achieve that effect. A normal lens (like a 50mm) would balance the two.
In the example, the first example would emphasize the fountain - which would be great if it was a beautiful fountain. If the fountain was boring and the building beautiful, you'd want to emphasize the building, and finally, if they are both great, you'd use a normal lens.
Being able to use focal length perspective to emphasize subject is one of the most important skills you can have for commercial work.