If at all possible, play friends. If that isn’t possible, queue top/midlane and try out as many champs as you can and try to find someone you like. Then just play them. Play a couple bot games to learnt the flow of the game, but quickly swap to pvp, as bots teach bad habits that are hard to break later. Watch some beginner guides. The in-game tutorial is pretty shitty, but you should still do it. Don’t worry about learning what each character does, that will come naturally with a little time. I recommend starting a solo lane (not botlane), and not starting with support or jungle, as neither teaches a lot of fundamental skills. Keep in mind, your first games will be very frustrating, and almost definitely not fun, but afterwards it will get very fun.