There’s a subreddit and discord called Summoners school. Going to drop the discord link below as a lot of us are on lemmy to avoid Reddit.
Mobas are hard because of fundamentals people know and you don’t. Learning some of the basics is a huge step up. Tons of YouTube and guides on summoners school will help with that. Don’t worry though too much about picking the best champion. Below emerald ELO (probably even after that), knowing fundamentals and really knowing “your” champion is a bigger deal. Pick a role you like. Then pick a champion that appeals to you playstyle wise within that role.
Finally don’t let failure get you in a negative headspace. It’s really easy and happens often where you are playing against champions you’ve never dealt with before. If the opponent knows the matchup, odds are you get spanked. That’s okay. Review each death and just note what you could have done different and the next time you play that matchup it will go a lot better. League is a game of who has the most experience in a particular scenario.
Take your time. Push your limits and don’t be afraid to die. People get stuck with this “play safe” mentality and you end up in a lot of games where people miss opportunities because they don’t want to risk a death.