Without seeing you play, all I can offer is generalized tips:
Learn to last hit.
Learn to last hit under your tower.
Let no minion go to waste.
Learn the timers of when creeps respawn (dragon, baron, Red/Blue buff), and call out in teamchat when they'll respawn right after you kill one.
Ward and get rid of enemy wards. Sight is power. Even a 0-17-0 Vayne can be useful if she sticks a ward in every bush she comes across.
You've been playing for two years, so this is probably moot ponit, but I'll say it anyways: Surprises kill you. I don't just mean people ganking you. Know what your enemy champions do. Have an understanding of how long their cooldowns and ranges are. Know how quickly and effectively they can burst you down.
Vary your build. [BerserkerGreaves, PhantomDancers, Bloodthirster, Infinity Edge, Black Cleaver, and Guardian Angel] is not as effective as Tossing in a thornmail vs an AD heavy team, or a Maw of Malmortius vs an AP heavy team.
Don't be afraid to lane swap. It doesn't matter that you win your lane if the other two are losing. Get your fed butt down to bottom and help them out. Show that solo top a thing or two. Remember that it's a team game.
Mid-late game, large packs of enemy minions march towards your towers. Think to yourself "who needs that pile of gold the most". Then think "Are they in a position where they can get to that minion wave in time".
The Baron is a tool to finish that last team fight and win the game. If you've got a couple inhibitors down, leave 1-2 in base to deal with the minion waves, and try to ninja the baron before the enemy team can. Or at least make them regret trying to take it.
Most important rules:
Don't get mad. You'll make poor descisions, and it can ruin the game for you and others.
Communicate with your team. Especially before game.