Disclaimer: I'm not the greatest at positioning either.
However, a tip that I have been given is to shoot the person that you can reach without endangering yourself. Don't stand close to an enemy tank with CC because people are yelling at you for 'focusing the tank'. Otherwise he can probably lock you up for his damage dealers to take out.
ADC's are more for high sustained damage than bursting someone out anyway, which means you're best suited to kill beefy people. If a mage tries it for instance, their combo won't kill them and then the mage just stares around for a while.
Finally, part of positioning does sort of depend on map awareness. Is someone on the enemy team missing? Are they trying for a flank? Make sure to ward possible side flank routes to protect yourself and shy away from being in the back near a dark part of the jungle.