I suggest you cotinue practicing in bot or custom games first and foremost. You have to get used to the idea of last hitting creeps, which is basically waiting to shoot them when they are at near death for the gold only. While doing this you need to move back and forth so you do not present yourself as an easy target and so forth. I only state the above to make sure that you have heard of the concept and not to make you / me sound like an idiot, but sometimes people do not know what they are asking.
As for an ADC choice, EZ is great, but has a higher learning curve because he has skill shots, as does Kog. You have to practice with these champs to be good and ensure you are hitting those shots by landing them on a champ versus having a minion shield your enemy.
With that said, you may want to pick up a champ that is easier to learn the basics of, and learn the role. Most ADC's have some form of AS boost, as well as some form of AD boost. Look for those that fit how you play, and will be easy to learn, maybe focusing on those that you can easily "forget" about a skill and still survive. Graves has that all around ease of use, in that he has his damage and speed boosts, plus he has a slow / blind and a mini flash due to his dash. His Ult is a skill shot though, but otherwise fairly early to on strong and easy to learn. Other ADC's to look at are Miss Fortune and Tristana, for ease of learning. Trist has great lane control/push through her E passive exploding minions on a kill, and has a great escape or charge mechanism with her rocket jump. Miss Fortune gives you a lot of easy utility, but is acceptable to ganks because she only has a slow, that takes a second to get off, so flash or ghost is required. However, her abilities are straightforward, and her Ult is one of the easiest to use for great team damage.
If you want to learn Corki though, you just have to practice. He is a tremendously tough ADC, but again, you need to learn when to use your abilities. For a learning player, the biggest issue is thinking that as an ADC you can just kill everyone quickly, and thus making the mistake to dive when you should have just settled down a bit. Read some basic guides, even if outdated, as they will give you hints on how to be successful as you find your groove.