how to know that a particular song is in particular scale and how to find out wat chords to b played?

hi guys am chaitanya i've learned playing guitar online nw am capable
of playing songs by watching online lesson youtube and all buh my
question is hw can i know a song is in particular scale and wat chords
to b played and how can i change the key or scale of that particular
song and buy hearing a song how can a guitar player knows that it is
in A or B or C....scale
PLZ help me out am unable to get answer for this :(