Most of you have probably seen this handy site before, but for us uninformed few it is still a handy learning tool:
Online Depth of Field Calculator
Just don't get too carried away by all this geeky calculation business, simply learn what you can then get out and shoot photos in real everyday situations. It's still the best way to learn!
(Although it's kinda fun to release the inner geek just a little bit! Haha!)