How To Gain Weight? I Am A 25 Year Old

How to gain weight? I am a 25 year old male but I m Lower in kilogram my weighs is 58kg and my height is 175cm before 2years I got upto 64kgs but now it's gone and before back in the years I drink smoked daily but now I quit all I do Ecg, echo,tmt as an aware of heart attack but now I quit all pls say some helpful things to gain weight