A great general tip is to be positive and absolutely communicate with your team on all levels.
People mention I talk a lot or too much and then they shut up when I say I want to win, dont you?
Always use your pings, ping for everything objective wise, same for helping your team focus down champs.
Another thing, map awareness map awareness map awareness. No joke. Pay attention to enemy locations for a quick dragon or buff steal, push lane and take tower because the jungler is top and mid is in their lane.
Vision. Dont be afraid to buy a ward or 2. Make sure to use your trinket ward. Late game turnarounds and plays are made off of vision. You can take a tower in 15-20 seconds w 2 champs and safely because you just saw most of the enemy team on the other side of the map.
Play to your teams strengths, what I mean is, if you guys cant win a teamfight DONT GROUP. If the enemy groups pressure multiple objectives to pull them off and only tf under turrets. No sense feeding aces til you die.