PSA: Your build is important
This isn't a thread attacking off builds, so let's just knock that out right now. No, this is a thread about learning how to build.
Every person has run into at least one player that does great - just excellent - in playing his champ. All the outplays in the world, all the calls, just a legitimately good player. And if you think you haven't, you might have been playing against him.
And wherever you're placed at, you probably looked at that guy and said, "Holy shit, why is he in this MMR with me?" And maybe he's just having a really good game.
Or maybe the reason is because you take a look at his build and he still hasn't even bothered to pick up a Last Whisper for 1300 while he's the ADC versing 4 tanks all building thornmail. He also hasn't built any MR whatsoever to counteract the collective magic damage.
This clearly isn't always the case, but you know that guy - the guy that doesn't have a clue what to build, when, where, and why. A lot of people don't realize that it's not just important to know how to play your champ but it's also important to know how to build them as well. And that doesn't mean going on mobafire and typing in "How 2 bild Yaso kek". While that'd be a very easy way to learn a build, those builds aren't end-all-be-all. And as much as I'd like to explain to people a formula or the formula or some by-the-books method to build, it can be a long, complicated, even convoluted explanation. So instead, I can tell you how to start learning how to build for yourself.
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You do need to learn your champ's general build. This doesn't necessarily mean a specific set of items so much as what your champ needs - for example, if you have very long cooldowns, then CDR is something you'll want to look into for your build. If mana's an issue, items that address that will work. What items address what? Well, that brings us to
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Learn what items do. It'll take a while for you to memorize the entire shop catalog, but you don't really need to remember each and every item - however, if you want to get back to lane faster, you will have to remember what items are important to your champ. For example, let's say you're a tank - there's a fair number of tank items to choose from. You really don't need to remember just what Death's Dance is and what it does or what Runaan's Hurricane does for you, but you should remember what Deadman's Plate, Randuin's, Thornmail, and Frozen Heart have to offer as well as Spirit Visage, Banshee's Veil, and Locket of the Iron Solari. Knowing the items and their approximate stats is important to make the right call at the right time (though currently Deadman's is pretty much all you really need to know for armor) and it saves you a lot of time browsing the shop in-game, which means you get back to lane faster and get more farm as a result. Also: read the damn item patch notes. Even more than champs is realizing when a change has occurred that could affect how you build and what your goal is. For example, if you're going blue build Ezreal, you probably don't want Essence Reaver anymore since the mana regen only procs on critical strikes. So unless you're building a lot more crit (and with blue build, highly unlikely until much later), this item is vastly outmatched by a myriad of other items that offer the same cooldown reduction since you only get more CDR if you build more crit items. So without other items, you're looking at a passive you're probably not using with decisively okay stats versus, say, even Youmuu's that offers 20 armor pen, increased MS, and the same cooldown reduction with the same AD increase. Staying informed means optimizing your build, folks.
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Learn your opponents; build to your opponents. What I mean by this is the first thing you should check every single game (after making sure you have your correct runes and masteries) is what the enemy team is. This can and should affect how you build during the game. This is the number one thing that people ignore. There is no one single build-path that works - sure, you could say for ADC's your final build is generally almost unchanged from game to game. However, the way that you get to that point should be noticeably different across a vast number of games. The reason for this is because you are more than likely facing different champs and different compositions - even as an ADC, the way you build is going to be different between if you're going against 2 assassins, a mage, a support and an ADC than if you were facing a team of 3 tanks, an ADC, and a mage. If you haven't invested the mere 1300 gold in the latter game to get Last Whisper rather early, you're probably going to be stomped in the face - meanwhile, if you tried getting that in the former game, then you're going to look like a doofus as the enemy Zed/Fizz/Akali/whoever mauls your face. This is the same for any role. Your opponents do dictate your build with their champs, and that's how it's supposed to be. Make sure you know what is required.
Once you've started doing these things, you should find yourself performing much better than usual (you know, unless you were already doing this). You have to remember that it's not just flashy-plays and high skill that LCS players focus on - it's also what the hell to do with your gold, and spending it wrong can put you further behind than anything else. Make sure you know what you're doing when you go into ranked, and good luck.

Nobody will even understand what you're doing. How do you combat someone who isn't building normal items but still dealing insane damage? The enemy then has to build around your own build, which people pretty much never do.
Which is all well and good, but you can hit the 45% CDR cap (if you're running deathfire on Lux you're doing it wrong) super easily if you take into account her passive and build around it:

and

eh what do i do??
[slayer-jinx-unamused]
counter accomplished

If this isn't your build, then L2playnoobs
If you can't get this build you will be stuck in bronze with me
gets flat m. pen
might help
as
when going up vs an all squishy team is a fine thing?
for instance, and we all know how useless tank gp is, and if their jg is squishy, I always go for