so ,,
am a sucker with theory ..
I dont really think am fine wit scales which is basic xD
but boy i improvise ...

but my question is how do i knw which key the song is in ?!
I never paid a dime to a teacher to teach me guitar. Coz i wanted to do it all myself ...
Can please some1 help me how u just make up ur own mind after u listen to a song and jus say oh its in B ?!

Well find out the chord progression, usually the first or last chord in the progression is what key it is in. Find out that chords by the root note of each chord change.
Memorize all of the chords in each key, understand chord functions and their sound.