Top: Always pick to fit your team comp. If your team is a pile of squishy crap, pick a tank. Not good at any tanks? Go learn one before playing ranked. Run teleport so you can gank bottom lane. Bot lane is nearly always occupied by the worst players on your team.
Jungle: If you've killed a lane, push the tower with your laner. Killed bot lane or the enemy jungler and your mid/bot are still alive? Get dragon. Don't go "Oh, I killed that lane? I'ma go farm wraiths! :^)))))"
Mid: Play a low-skill assassin 90% of the time unless counterpicked and roam bot/top. Everyone gold and under has no idea how to deal with constant roaming.
Support: Don't play tanks in bronze/silver. You'll engage and your ADC will do nothing and you'll just die. Learn poke supports: Sona/Zyra/Lux/Lulu and solo carry your lane. 95% of ADC mains are absolute morons who can't position, ignore pings, and don't follow up on engages. With the aforementioned champions it won't matter if you accidentally take kills; you can carry. Spellthief's first into Sightstone always.
ADC: Don't play ADC.