Go to youtube,look up: laning phase, last hitting, team fight positioning as an ADC. Then look up a guide for your favorite champion .
Play custom bot games by yourself get a feel for the champs power (what you can do with few to no items).
Practice some build orders. Play a few co-op bot games then when you feel good about your ability with the champ go either to normals and practice against players or back to ranked.
In short, learn: the phases of the game, the factors of team fights, the mechanics of a solid champion for your role and then practice until you feel confident about it.