hmm.. how ar?
there's no sure and fast way to learn, except to get out there and
shoot.
know a bit of ur theory first.. for example, most know that big
apertures (small f number) produces a shallower DOF, but do u know
why?
then u will slowly pick up things like when shooting (still)
humans, 1/60 is good to freeze them, unless they are moving very
vigorously u might wanna go up to 1/125.
then u might wanna throw a flash in some time or other, and then
there's a whole new ball game, foreground bright, back dark,
shadows too harsh, etc etc.
basically.. just shoot lar. and learn from mistakes.