You're probably playing too many champions.
Pick one champion, and learn to play them well. Then pick two back ups that you play decently, one for when you get your secondary and one for when your main gets banned or picked. I would say the more versatile the better. Sometimes wins come down to team composition.
Another good one is focus on farming, not fighting. Anytime spent out of lane is not worth it. It doesn't ever matter how close you were to killing a champion, if you die, you're losing a lot of time and gold. You also won't be winning another trade with them unless they mess up or dive you. If you die once, don't try to duel them again. You'll most likely die again. One death is easy to come back from, two and so on is much more difficult.
Also know when you're playing against a champion that is especially oppressive when on a killing spree, so like Yasuo or Riven. You really want to take care against those types of champions, and make sure your teammates know when they're roaming.
Finally know your priorities.
Nexus > Inhibitors > Turrets > Dragon/Baron > Farming > Kills
Turrets and objectives like dragon are situational. It depends on where everyone is on the map, and which you're closer to. I will say I like to get first dragon when possible, and that usually happens after the first death bot. However if it's a situation where a tower is about to go down, take that first before going to dragon. Know that when you have a lock on your priorities, you will get more kills and win rather than just seeking out kills.
Grouping is important too, but in lower ELO it's left up to fate even into Gold. All I can say is don't be that player that's split pushing when the team is winning and fighting, and don't be that player that is charging into fights when the team isn't grouped.
Everything else you just learn through playing the game.