Hello! So first of all I think that you should be having fun no matter what with this game. Realize that that's all it is and it makes the playing experience a lot better. I think that you should really just buy champions that interest you and try them out. You're not gonna be a pro right off the bat and that's okay! One piece of advice I'd give you is to let go of past games. Don't carry the negativity that happened in one game into another because you'll end up playing worse and getting angrier at the game. And to answer your concerned about OP champions in the meta right now, Riot has actually done a pretty good job at balancing the game at the moment. Right now you don't see anyone crazy wrecking everything without a way to counter them. Don't give Darius kills, make sure to dominate a Vayne early-mid, build health against Fiora. Everyone that's really powerful right now can be countered. Also, in my opinion, there isn't anyone in league that can just face roll and get a penta kill. Even Katarina has some skill level needed. Granted, when certain champions get ahead, there isn't much you can do to stop them (Vayne, Darius, Fiora, Nasus). My biggest piece of advice would to be to go out and really learn about the game. The more knowledge you have of champions, the map, objectives, etc. the better equipped you'll be to face the challenges of the game. I suggest watching ScrapComputer's channel on youtube as he has, in my opinion, the best guides that go into great detail about topics that and champions that really help you out. I hope this helps you man. Feel free to add me, and I can try to help you with your endeavor in having more fun in this game! :)
