This is a very basic question.I am trying to learn how to use AV and TV modes with my T3i (after 2 years of shooting in Auto). What should I set my aperture to in order achieve the best results? I know how to blur the background or keep it sharp by changing the aperture value. However, what if I do not really have a preference ? I usually just want to take a picture of my kids. When I use the auto mode the aperture is set for me. How does the auto mode determine the best aperture value for me and how can I determine the best aperture myself in AV mode? Is this basically a creative choice (i.e. sharp background vs. blurred background) or are there specific rules for the "best" aperture value?
The same question applies to shutter priority mode. I understand that high shutter speed freezes the shot and low value will produce a blur. I would usually want to freeze the shot. Does it mean I should choose the highest shutter speed first?