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.
Learning by doing. This is how the mind works. Play a lot. Practice tactics about an hour every day at http://chesstempo.com/chess-tactics.html
Two extremists which I notice a lot. 1. The beginner plays too cautious. 2. The beginner plays too aggressive (king's gambit). As for openings. Forget it until you are more experienced. In your shoes I would probably play the London system as white and perhaps the Dutch stonewall as black against d4 and classically 1.e4e5. (Further explanation of openings is beyond the scope of this question). For openings I often use game collections at www.chessgames.com.
Speaking from experience I find that my play improves from physical exercise.
I want to address the advice from Scounged in his comment.
I myself have been studying the opening a lot from the very beginning. I played the Kings Indian and the Najdorf combined with 1.e4 main lines. But my tactics skills where lacking. Also, I walked in to unfamiliar positions all the time. Nothing particular bad with that, but, they were familiar to my opponents, giving me a vey hard time. Then I changed my repertoire. Then I changed my repertoire again. Clearly, what I should have done is to play 1.g3 or similar in all my games and focusing on tactics and ideas.
Unless your opponent also knows and are willing to play the main lines the positions you have been studying are not likely to come up in your games. My feeling is that opening books are addictive (to me) and is great for giving motivation (perhaps also memory training). Giving that 3 world champions reached the top without playing main lines I stand firm in the no opening theory section. The world champions are Alekhine, Capablanca and Carlsen. I do think they know the openings ins and outs and the principle behind them, however. Perhaps a middle road is better for some, but I don't think it's effective at all.
When you have reached a certain level the endgame is of outmost importance. Many coaches with ambitions students teach the endgames first. Endgames are considered the hardest part of chess.
One suggestion for a rather solid game is to play 1.d4 and the London system. When you have mastered tactics (several hundred hours) and are bored of London you go on with 1.d4 2.Nf3 3.c4 and 1.d4 2.c4.
One doubt is that... If I study one opening ... For eg... Ruy Lopez... But opponent plays something different... Then I can't continue with ruy Lopez system... So is there like a bunch of opening system which you find it useful? Which I can read
One relatively quick solution would be to select a repertoire where your opponent's choices are restricted and you can follow some kind of formula to get your pieces developed.
One example would be to play the King's Indian Attack as white and the King's Indian Defence / Modern as black. That way you drastically limit what you need to learn and over time you will find that you become more and more familiar with the positions and plans that come out of these openings and so you should be able to get good results and avoid unpleasant surprises in the opening.
I'm not suggesting that this would be a particularly exciting approach just that it would be effective. I believe there have been quite strong players (IM / weak GM standard) in the past who have adopted this kind of strategy. It is not one I would recommend to a junior (school age) player since they have the time to study more widely and become a more rounded player.