Evaluate the targets, and do constant damage to whoever is closest. Stay at a safe distance, kite when needed, and pray to God your team peels for you. In the case of two similarily distanced enemies, deal as much damage as you can to the bigger threat.
Some of the MANY Things to consider when deciding:
Who's got a gap close?
Can they deal damage once they're on top of you? Or is the other person more dangerous as a follow up? Parallel to this, can you kill either of them with a single autoattack? Do it.
How fed are each of them? Is the typically weaker champion way more fed, and therefore the bigger threat?
Do you have a meat shield in front of you thats tanking skill shots?
Can the enemy sona flash-crescendo? Stay out of that range, or have your own flash ready and extremely quick reflexes.
is the whole team disabled for a few seconds? (Nami bubble, yasuo ult, etc.) you can move forward a little bit and maybe chunk the enemy carry in the back. But back off before they're able to move again. Chain CC on your team makes his much easier. Sej ult, followed by Nami ult, followed by a yasuo ult disabled them for days and there's no reason you shouldn't get some kills out of it.
Typically, never ignore the tank if they're the closest. Your job as adc is to put out as much dps as you can while staying safe. You have to rely on your teammates to peel the voli or singed from flinging you, the jax from stunning you, the Chogath from feasting you, the list goes on and on. This doesn't mean you stand still while your team peels them off. You kite them while they're being peeled. Once they're out of the fight your safe to move up a bit again, and get the next closest target you can hit.
Something else to consider is if you know for a fact that the enemy team has no vision of you and might not know you're there. Stay out of vision until the gap closers are down. You're against a jax? If you're hidden, maybe he'll use his leap early and jump on someone else. Once his jumps on cooldown, you start hitting as much as you can because he has no chance of getting to you. Don't wait forever, and don't reveal yourself to early. It's all situational, and you'll have to have really sharp decision making to do the best thing
http://youtu.be/AHQ0mB57UX8
There's an AD carry section in the video. Find it, or just watch the whole video. The whole video will give you an idea of what to expect from everyone in your team comp. team fighting methods vary based on the number of tanks, assassins, etc. on each team.
