dude do whait iv told u
eat 5 times a day , not big meals ,, just normal sized meals ,, have protein in each meal
n exercise regularly with medium to heavy weights
12 kg is nothing ,, ul gain it soon ,,
i am 6'1 and i was 60 kg now im 86kg
all these guys keep sayin eat eat eat ,, eating wont do anyhting cuz of ur metabolism ,, u will need to exercise ,
eat good ,, exercise regulary n sleeeeeep is must cuz ,, ur muscles repair when u sleep
also ,, when u exercise with heavy weights ur muscles fibers get torn or damaged slightly ,, n when u rest the body repairs em with thicker n stronger fibers , hence u get bigger muscles wiht time