Knowing what you can get in the game and understanding what the enemy is building. How to buy items efficiently against the your opponents?
Getting a guide online on how to build your champion is one thing but being able to counter build off your opponent is just as important as knowing your abilities. If you just build the same way every game you may be losing out on the overall of your champions ability to kill your opponents. Example if you are playing a tank top you may need to build armor one game and the next game you may need to build magic resist (MR) to make it so you can absorb more damage to stay alive longer in team fights. This is counter building based off the opponents champions in the game you are in.
If you are new to league of legends knowing what items to buy may be difficult but if you can try to understand what they can do and there stats they give, you will be able to know whats more effective in certain situations. Items are what can make or break a champion so picking the right ones is key for a successful game, it is also important the order you buy your items in. Example if you buy boots as your last item you will be slow all game, and may get caught out because you are so slow.
You are able to buy a small version of the item you may be trying to build, this makes it so you don't have to wait till you have all the gold at one time to buy for the item you want to buy. Example you can build an "BF sword" and then build into a "bloodthirster" and you can also combined two items to get one big item you will receive an discount worth the same value as the two item you bought are worth. There are only six item slots, this means you can only have six items on you at once, potions and wards take up an item slot but tickets have their own place so it does not take up an item slot.
To customize your items selections to show up when you are in a game, all you have to do is customize it before you get in a game. Open up league of legends, then click view/edit summoners profile at the top right, then click item set at the far right, for there you can pick the items that you may/will buy during the game. Just like your runes and masteries you can make it for a position or a champion and you can also read up all the items without being in the game letting you know ahead of time what may be good.
Note: Riot games is always adding or removing items in the game and in some cause they change existing ones by "nerfing" it or "buffing" it. Reading the patch notes will keep you in the loop of what has been changed, or removed/added.