Most questions of this type have been thoroughly and comprehensively covered in old threads. Search in the "Theory" section, if that is possible. Noticingthemistake is pretty much the resident guru of theory, so check his comments out carefully. But there are lots of other people here who know their stuff too.

But yes, the key of a song is usually the chord that it begins and ends with, though there are exceptions. A chord is made up of notes--usually three.