Here are a few things to not do in order to become a better ADC.
1. Focus farming.
It seems obvious to everyone to prioritize gaining experience versus poking the enemy. However, this is a mistake we've all done (myself included). It is better to focus on farming since it makes you stronger than your opponent and get items before them. Doing so gives you an advantage. If the enemy is poking at you, ignore them. Your support is going to do the poking in return and ensure your safety.
2. Keep an eye out on the map.
Every player should have map awareness but as an ADC, it is extremely important since you are squishy and easy to kill. It is important to be aware when your jungler is coming to gank (or your mid-laner if they are roaming) and get a kill from the enemy champion. Or the opposite, keep an eye out for the enemy jungler.
3. If you are losing...
...get life steal. It is the only potential way of winning over the enemy ADC. It is not necessary to get life steal just when you are losing. Your goal is to remain alive as long as possible to push towards the nexus. A bloodthirster is good for every ADC since it had damage and life steal. In addition, a Guardian Angel can be good as well. A Guardian Angel is not a life steal, but it is an armor that can revive you. Stopwatch is good too but ADC's first item (or secondary) to build is the Bloodthirster.
4. No Solo Roaming.
I cannot stress how many ADC's do this mistake, but they roam alone and get ganked. Yes, your role is to push but if you're not fed and not strong enough, do not overextend yourself in to enemy territory (unless your team is distracting them). Overall, ADC's, be safe.