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For starters let me mention, that if you have an account on youtube there are two channels that I find really good and am subscribed to.
Riot Games has a channel "RiotGamesInc" It gives regular updates on client patches and has the so-called "Champion spotlight" where they focus on a champion, tell you what it can do and gives hints on how to play him and how to play against him.
The second channel is Protatomonster and what he does is more for the community. He uploads special gameplay videos and lets the community decide on which they liked best. This includes awesome teamfights, lucky plays, fun baron steals, and much more. So if you like to get involved in the community this is one of the ways.
Some links I find useful:
1. League of Legends Wiki
2. Summoner School
AD Laning Guide
After this video I got ganked a lot less!
Also, useful if you actually want to play well, watch live streams and gameplay videos.
I am always watching videos on the champion I want to play.
The championships are interesting as well, just to see how world class players play. From those you will learn a lot of stradegy because, just like in other sports, there are commentators that tell you what is happening and explain why people are doing what. Helps understanding things a lot.
The only thing I find difficult at times is to find out where to go and what to do after laning phase, because at that point I never know where the enemy champions are.
So if anyone has tips for that, let me know.