I played League for about five years and just quit recently.
It is an incredible game that took me years to be able to play at a basic level, and I can't even say I was "good" at the game when I was done with it. When I look back on my time with League, it blows my mind how such an astronomically popular game can be so unintuitive. Items, jungling, when to go for objectives, who to play in what lane, a cast that is closing in on 150 characters -- there are so many facets of the game that you cannot understand without sitting down and studying how they work for hours upon hours. And even when you have raw knowledge about what items to buy or how a jungle path works, it is very difficult to convey to a new player the "flow" of the game, or how they should react in certain situations. When should I fight? When should I flee? When should we be going for dragon? When should I stick with the team or go off on my own? Can I rescue my teammate or should I let them die? PUBG dictates its flow via a circle of play that rapidly closes and forces more encounters. Many modern shooters impose a time limit. With League, it's all on your team to make the victory happen, or else the game will continue forever.
Before you do something as simple as fighting someone in your lane, there are a lot of considerations: items, abilities, cooldowns, summoner spells, resources like HP and mana, how close you both are to leveling up, whether you have vision of your surroundings, how close your allies are, the risk of ambush, being able to dodge their skillshots. You have to build up your experience through hundreds of matches, going through enough situations until you build up a "sense" for how to act or react in different scenarios. Only then will your experience lead to familiarity, and eventually you can boil down all of this information into an instinct: "yes, I think now's a good time to go in."
Jmac has some good advice on the team aspect. I recommend learning the game with other new players, and also with an experienced buddy who can answer your questions on the fly. If you go in not with the objective of winning (though of course want to do your best) but the objective of having fun and learning more about the game, you will have a much better time. League has some of the meanest, stubborn, unreasonable, and defeatist players I have ever met. Through them, you'll also see firsthand the huge impact that psychology and negativity can play in the outcome of a game. I do not exaggerate when I say that one player's bad decision can cause a team with an enormous lead to lose it all, and it happens far more often than you would think. When you lose, you have to be very reflective on the choices that led to that outcome, and focus on what you, personally, could have done better.
Stick with it, and you'll find a game that rewards your time. The Rift is a playground of skill and creativity -- every inch of that one map has housed some insane and memorable encounter. Riot updates the game very frequently, so if you really want to dive into the game, you have to keep up. I've watched them get better and better at developing League's unique aesthetic, releasing super cool new champions, skins, cinematic videos, music, and lore. As frustrating as the community is in-game, there are a wealth of talented and creative individuals who celebrate and enrich the game from the outside through fan creations. Even the competitive side of League is a world all its own, full of history and stories and moments.
Anyway, just wanted to share some of my thoughts on the game. Good luck to all Allies who wish to try it out. Love and respect is a motto that will definitely serve you well in this game!