Your question is not simple, and it depends on your lane, but I'll try to help.
First off, sometimes you just get fucked by your 'teammates', try to not get frustrated. You have to focus and always do your best.
Focus on getting your cs numbers up. Gold wins games. If you are 30cs up on your lane opponent thats roughly 540gold.
Keep in mind what your jungler is doing.
If he comes to gank, try to make it worth it, bait, but be smart. For instance take a bad trade and let your jungle clean it up. If hes ganking another lane, back off so you don't get ganked when the enemy jungler knows he won't be counterganked in your lane.
Focus on where the enemy laners are. Don't rely on pings.
Is mid missing? Is the enemy support missing? When did you see the jungle last, where is your jungle? Keep in mind that if your jungle shows on the map, thats a green light for the enemy jungle to gank too.
Focus on your decision making. Analyze risk vs reward. Analyze the results of your decisions, especially the bad ones. Think about how you could've played differently in order to not make the mistake- in order to not make the mistake again. Make sure you are learning every time you screw up.
Try to set vision and buy before you get into a fight. For instance if drake is up in a minute, try to push your wave out and back. This applies to any objective. There is nothing worse than needing to be at a fight but having a significant amount of gold in your pocket. Being in a fight where you haven't spent your gold is like a bait on your teammates, think of it that way. If you aren't ready to fight, tell your team, ping how much gold you have and tell them to delay the fight. That is asking a lot, but its all you can do. The goal is to have spent your gold anytime before you fight. Which also means controlling when you fight.
The more gold you have accumulated the more cautious you should be about fighting. Just because your enemy laner hasn't bought, doesn't mean the enemy jungler hasn't. He could be coming off of a buy and be ready to one shot you.
The more effective you are at analyzing yourself, learning/applying changes, the faster you will grow. League is extremely complex, there are always things you can be improving on. Be sure not to autopilot or you are just wasting time.
These are general things that will improve your skill regardless of champion or role. Meaning these are things that overall make you a better player, and will be the most effective at growing your collective skill. League is about math, execution and consistency, not feelings. Feelings are how you go 0/5 in five minutes. League is chess, not checkers.
Some other things:
Go for outplays, try to play aggressive and push your limits. Don't play safe unless you have to play safe, sometimes you don't have a choice.
Trade if you are going to die. Meaning if its a three on two and you are dead, but if you flash and get off one more auto you kill one of them, do it. Get the gold. This also applies in a situation where you are going to die but teammates are on the way, use all of your abilities to give your 'teammates' the best chance to clean up if you can't get out.
Always work on thinking ahead. For instance if you are low under turret, but the enemy is pushing in a huge wave and you need to stay. Ping your jungler for help way ahead of time when you know its coming. You need to be looking 20-30 seconds ahead on waves. A minute on major objectives like drakes.
Mute everyone. Riot only penalizes you for talking, so don't, use pings. Fuck chat.
Depending on your role, peel your carries. This will win you tons of games. When teamfighting there are two methods, engage and protect. Engage is when you try to start a fight, protect is when you try to peel a fight. For instance an engage would be amumu going in and ulting the enemy team. Protect would be standing on your adc as mundo because you know the enemy has a talon. Play teamfights according to your teams strengths. If the enemy has a lot of aoe, you want to fight out in the open. If you have a lot of aoe, you want to fight in small spaces. If they have sneaky assassins you need to protect your backline. If they have hooks/picks/rundown you need to focus on vision control, so on.
Thats enough for now.