I've just given this a quick go in the last few days and it's surprisingly addictive. Any tips outside the usual you will see starting out? The buying process seems pretty complex.
I would suggest not playing ADC. Its not a nice place to start as a newbie. I would suggest playing Top lane, then either Mid Or Jungle. Playing Jungle will give you more time to understand the game.
Focus on mini map awareness, at first its really hard but learning how to switch between laning and paying attention to the minimap is a skill in its own right that is often forgotten about. Also like someone else mentioned, last hitting is important so know how to farm correctly.
Regarding buying items and advice, it will come with time, but for now look at website like U.gg and Probuilds and Op.gg for buuying/builds for your selected champ.
Above all find a champion you enjoying playing and play it to death. you want to know the ins and outs of your champs, whaty ou are good at and how you play him/her.
Above all have fun and try not to tilt or take comments in chat to heart.