I see a lot of strategy posted here, but I think that stuff is somewhat irrelevant until you hit around 5k. Reason is, so many games are won more by teams being cooperative and aware of each other's situation/intentions than actually knowing "how to build X carry"
That stuff is important, but it first requires good mentality. I see games lost because people are petty and getting mad, not because they didn't understand how to play the game. Short story time.
When I was just a 3k player, I was in a game with a storm. Our support didn't buy words, just the courier. He was stuck solo supporting, and probably grumpy. So our storm starts feeding mid with no vision. The simple solution is that our support should buy a ward. It's just 65 gold, afterall. But instead our support wanted his boots real bad, and so storm continued to feed. And the game was eventually lost.
Who's at fault here? Our storm mid? Our farming, static jungler LC? The offlane necrophos? Our cm support? Or me, the carry? In reality, it's everyone's fault.
You can say some people have more fault than others, and that might be true. But the end result is that anyone could have done something about it, and that might win us the game, which ultimately is what matters.
Our jungler was greedy, weakening our lanes and not buying support items. Our offlaner was greedy by making our support a solo support, not helping with dust/ward purchases. It was our storms fault for not just buying a ward for himself so he wouldn't feed, and the cm support was also greedy wanting boots despite not really needing them.
But it was MY fault too. And I can control my actions, but not my teammates'. So I should have bought a ward for him. It might be unfair, it might be less efficient than the support buying it. But it's what's necessary to win.
This is what I mean by awareness. My carry build order doesn't matter if our mid feeds like crazy. I should be aware and help out the current situation.
This also means constantly checking your teammates items and intentions. In another game, I noticed my support was almost level 2, so I let him grab the bounty rune. However, in the game after that, I was almost level 2 but my carry grabbed the rune anyway.
It doesn't matter who should get the bounty, the point is that you demonstrate to your teammates that you're willing to work together and that you're being considerate/not greedy. I started noticing this all the time in my 5k games, where my teammates were oblivious to each other. I see it less now in mid 6k, but one time a lvl6 support bought the tome of knowledge for himself, meanwhile the other support was lvl4. The lvl4 support needed that, but the lvl6 was just oblivious and was thinking only of himself. That shit matters way more than a proper build order, whether or not a support has his ultimate before a big fight.
Finally, be more careful with how you word things. You don't have to be super nice, but some comments are just unnecessary. Either they take too long to say something simple, or they degrade team moral. Watching my team feed in teamfights and in trilanes against bristleback is annoying. Instead of explaining the science behind why it's retarded to chase a bristle back, instead I just say "split push, avoid fights." Short, straight to the point, doesn't allocate blame. Run away from enemy heroes, we have jakiro, kunkka, and underlord, we should be able to push out all lanes.
If you take a good 30 seconds to explain over the mic how bristleback works, you're just going to annoy your teammates, even if they are at fault. The point is to win, not get in a fight over your ego.
that's all.