Learning chess theory I have played a lot of chess in my childhood. And I'm quite a decent player as well. However I stopped playing for few years now. Only after I realized that there are lot of opening/middle/closing text book moves like 'kings Indian defense' etc. if I start fresh once again, how should I start learning these text book moves and implement it? There was no one to coach me in childhood so I never knew all this.
When you state that you are quite a decent player, exactly what do you mean by that? Do you mean that you have been producing relatively good results in actual chess tournaments, or do you mean that you usually win when playing against casual players? If the first description fits you, then studying opening theory may bring some value. In this case, you should try to understand why the moves are played in the opening instead of just memorizing variations, as the latter won't prepare you if your opponent steps out of theory. Look at famous games played in the opening and reflect over the moves that are played.
(Edit: And as r4 pointed out in his answer: don't be afraid to play the opening a lot!)

In the other case it will probably be completely pointless to study specific opening theory, as there are other things you need to learn first to become stronger. In this case it's better to review the basic opening principles about development, center control, piece activity and king safety, and follow them. In the vast majority of cases they are sound enough to give you an advantage against weaker tournament players, if you master them.