In my opinion, the best way to start your lore journey with League of Legends is to watch videos "Story of League of Legends explained" and "The world of Riot's MMO is already done" by RU-vidr Necrit. He is basically the lore master of League of Legends. He also has video that explains the lore of this cinematic too.
u should watch 'Story of League of Legends Explained' by 'Necrit'. from this vid you will able to get a general idea of the world of league of legends.
League has fantastic cinematics, beautiful music and incredible lore. You will not regret getting into it and you can basically start anywhere you want. As others have said, you can't go wrong with checking out Necrit for League lore. The Call is another top tier cinematic you won't want to miss. For a favorite personal recommendation I go with Xayah and Rakan: Wild Magic, which was released as a teaser for two champions (lovers) being released at the time.
1:20 If you understood Portuguese I would recommend you "História Contada" which explains all the lore behind the cinematics, hqs, website, etc. I don't know of any equivalent in English.
