First make sure your running solid runes and masteries to be able to last hit better. (ADC=15-13AD/21/9/0 recommended)
Practice with Ashe. Her playstyle is reliant on stutter stepping because of her permanent slow effect on her autos. Re-positioning after every auto with Ashe will get you into practice with other champions.
Go into a custom game and make a 1v5 bots. Go to lane and practice last hitting under turret.
Some tips:
melee minions= 2 turret shots+1 auto
ranged minions=1 auto+1 turret shot+1 auto
Don't be afraid to use your skills too. Volley+autos is crucial for last hitting multiple creeps. Just don't spam and waste your mana.
Practice farming and harassing the enemy bots by positioning yourself in front of a dying creep and volleying to where it kills the creep and damages the enemy too.
Stuff like this applies to other champions too. Ashe is a go-to champion because she teaches multiple mechanics in the game (Map control with her E [knowing where the enemy is or predicting where the enemy jungle is at] and global ult [changing a 1v1 to a 1v2 across the map], stutter stepping, skillshots, and positioning). She's an ADC and a Support at the same time.
There is other champions like Vayne or Lucian but their playstyles are more reliant on team comps and skill level. They have higher potential for outplays in teamfights but can easily be overwhelmed if behind. Ashe at least some safety net to fall on if she's behind by supporting her team with timed ults and giving vision on the map while attacking from a safe distance with her team.