I learned a lot just reading and interacting with other photogs on
http://www.dgrin.com/ which is a photography forum associated with Smugmug.
EDIT: Stay engaged right here on EXPO in the photography section and ask questions. There are several outstanding photographers that are happy to share their skills and are active contributors right here. :elkgrin:
From my perspective this is how I use those function settings.
AV = Aperture Priority I use this 80% of the time. You set the F-stop for the aperture setting you want and let the camera figure out the exposure. You still have control over your ISO setting if you need to tweak it for the conditions. Closed f-stops like F.16 for landscapes with large depth of field - open f-stops like F2.8 - F5.6 to narrow your focus depth of field to make a subject focus pop against a blurred background or you just need to be that open due to low light conditions.
TV = Shutter Priority I use this when the shutter speed is critical like when shooting moving wildlife or sports. Set as high a speed as the light conditions allow and let the camera figure out everything else. In this mode the camera will drive the ISO way up if it needs to to keep the set shutter speed so pay attention to that if you don't want the noise from high ISO settings.
M = Manual (duh) I only use this when doing night shots and I need to control long exposures (some cameras have separate "Bulb" setting for longer that 30 seconds).
P = Program Don't know, I never have used this one. :sombrero: