how and where we should use "who","whom","which".

You'll probably need to do some swatting on relative pronouns to get your head around it, for any complete answer would be a tad too long.
'Whom' is the objective case of 'who', e.g. if someone did something to someone else and you wanted to get to the bottom of it you might ask "Who did what to whom?"
'Which' generally refers to non-human nouns, e.g. "Which car did you drive last night?"

There's a lesson on who and whom here and one on that, which and who here.