Originally Posted by bronco
Right now I can go to AV Mode and adjust the F and the ISO, but not the shutter, or I can go to Tv and adjust the shutter and the ISO, but not the F....
A good thing too, at this stage.
See my explanation above regarding the proper exposure. When you adjust both shutter and aperture, most times you will either under-expose, or over-expose the picture - but luckily, these are easy to diagnose and fix.
This is analogous to pushing both the gas and brake pedals at the same time on your car.
Another whole aspect of DSLR ownership is to shoot RAW. That allows you to change exposure, white balance etc. after the fact during post-processing, but requires the use of specialized SW to convert from RAW to JPEG. IMO, once you are past the basic experimentation, RAW is the way to go.