ADC are generally Marksman, Ranged champions. like
etc.
They are generally squishy and weak early game. That is why you send a support with them to carry them through laning phase.
their objective early game is to farm Creeps by last hitting them so as to build items.
Getting early kills is also very beneficial. You want to get fed and fast.
The majority of ADC's builds center around attack speed, critical strike and life steal.
Their main damage output is just AA (Auto Attacks.. this is just a basic attack by right clicking on the enemy). Their abilities usually focus around auto attacking such as gap closers, disengages, crowd control, dashes, damage buffers and finishers.
It is generally bad for the ADC to fail behind early because it takes them a while to recover.
One thing that not too many people mention is that you need to get good at kiting or juking. This is the art of avoiding incoming attacks while counter attacking or counter acting to it quickly. Juking is more just the art of evading attacks. ADC's usually remain squishy through out the game so they become a high priority target for the enemy. You need to get good at avoiding all the incoming attacks and attempts to lock you down.
In team-fights stay centered if not to the rear before the fight begins. If the enemy can initiate on to the ADC or lock the ADC down,.. then they know the ADC will be in the thick of the fight and probably die quickly. You don't want that to happen. Wait for the initiation from either your team or the enemies. You generally want the enemy to burn up some if not all of their crowd control and AOE abilities so as not to soak up too much damage. However waiting is within reason. ADC brings a hell of a lot of damage to a team fight so you don't want to wait on ceremony either. Good judgment is required for when to engage and when to hold back.
Red buff is very useful to a ADC because of the after effect on hit targets. The affect also works on minions making the ADC very effective in pushing lanes quickly. This can also be useful for split pushing.
There are plenty of build guides on the net and you should find some to help you figure out what you need to succeed.
Also worth watching some YT videos to see how the pros do it. Can always learn something new.