Top, adc and Support.

Hello, I am nevis, I main Riven, and you know the lanne that is the less played? Bot lanne (cause of the supports and assassins btw), and sometimes I get autofilled bot, so I am Main Riven, I don't have the Knowledge of a Good Adc Player, It's really hard for me to cait because I always played league CLICKING in the enemy to attack them, but when I see people using Shift+M2 I am like "How did they do this?" I would love some tips to learn how to cait and learn to use the Shift+M2 properly.