Knowing the keys of music ?

emm...this is a rough area of music for me as i am only really starting on it. will somebody explain to me how to know what key you are playing in ? tell me everything you know about how you determine the key of a piece without being told ie: being able to determine a key of a random piece of music..how do you do that ? also place emphasis on how to know if your key is in the flat of a note or a minor key plese..i really want to get a hold of this stuff :)