I'm not sure if you've come across this already, but Camera Sim is a website/online tool to help you grasp the basics of photography.
Not sure what difference it makes when you shoot at F 2.8 vs F 22? You can see it happen before your eyes. Click the photo to be taken to their site.
p.s. This isn't an ad for them - just the most helpful thing for beginners that I've seen in a while.
I had to share!