Santos touched a critical point. The biggest bridge between an inexperienced and a more advanced player is judging the situation, being flexible and adapting your style/build. This is something difficult to tell in words but generally can be discribed as knowing your limit. Pushing exactly as far as possible, IWTHOUT overdoing it, because that would mean to die. You need to be able to evaluate a situation with high sertainty and engage as many favorable battles as possible, why disengaging when you feel at a disadvantage.
You should be able to take advantage of an early enemy weakness and push towers. Also, you should be able to adapt a more defencive style/item build if you feel things are going bad. A lot fo players stick to their primary set without taking into account that things have changed.
This is something that takes time. But if you want to learn faster, try to do this: everytime you die or loose an inhib, think about what happened and what you could have done better. Try to find your mistake and mind it next time. After each game - no matter victory or defeat, take your time (5-10 min) to look at everyones build, scores, to think about what went good and what went wrong, which mistakes did the teams make, what decided the game. If you really think about thoose things, you'll learn to find game changing elements and to exploit them.
Generally the thing that describes noobs is that in a fight, they can't localise the exact source of damage and the crucial factors. This doesn't last too long though. However, in a smimilar way, people can't tell what exactly turned the game and when. Try to put your finger on precise elements - this push was fatal, looosing this guy before battle lost us the game, we gave dragon too often to the enemy team, we were overrun by people with rune buffs, game was even but we kreeped and they took nashor and won, etc. Knowing exactly what makes a victory will allow you to take advantage of it.