I would suggest practicing in custom games for jungle routes and last hitting minions. Watching guides on animation cancelling. Watching some pro streamers. Try reading guides for the champions you like to play, not one, several. Also try out a variety of champions and roles so you better understand them when you play with or against them.
Then pick a main champion: I main Ashe. And some secondary champions: Riven, Leona, Shyvana and Teemo for me. That you want to be good at. Practice them a lot, learn their specific mechanics well. And make them your primaries. Still doddle around in other champions from time to time, to keep your perspective fresh.
Always read patch notes. And try to keep up to date on the current meta.
To see who is currently good and bad in the meta, and who is being picked or banned. Go to Champion.gg
Start to analyze how you play. Think about your patterns for dodging skillshots. Is it erratic enough to not easily be predicted? Think about how your enemy dodges skill shots and how they react to your actions. Can you predict their patterns and strategies?
Ping everything. Missing enemy? Ping it. See enemy jungle near your lane headed towards a lane? Ping it. See an enemy use a long CD summoner spell or ultimate? Say "Blitz Flash" - "Ashe ult" etc. In chat.
Work with your team, not against them. If they need you, come to them, if things are sparse, farm, push.
WARD! Regardless of what lane you play, or role. Ward, key points. On cooldown if possible. Always pump out all the wards you can. Even a few pinks.
Try to keep a good eye on your enemies and allies build, and damage composition. Counter enemy items and champions by your strategies and build, while complimenting what your team needs. Your build should never be static or preplanned. It should be proactive and reactive.
Take more risks and play hard champions. Even if you look bad at first, laugh it off and have fun. You will learn what works, what doesn't, and earn more courage.
Do not be afraid to admit you CANNOT win. When I am Riven against Illaoi, I just straight up know I am not winning that lane. Unless Illaoi is a potato. So I focus on farming safely, teleport ganks and scaling okay into late game. Doing my best to deny her kills and not let her catch my team off guard. I am not going to dive an enemy with a gold, mechanical, skill or design advantage over and over in some sort of furious 'bravery' that is foolishness. Don't be afraid to play it safe and play catch up.
DON'T fear the meta. You can break it, but breaking it isn't some noble deed. The meta exists because it is general what is effective, right here, right now. Following it can be a path for success until you have the skills to come up with occasional off meta plays of your own.
Most importantly, don't let people get you down. Play your best, have fun, ignore toxic comments. People don't really get "200 farm by 20 minutes" nor is everyone really going "20/1/20" every game. I've watched pros like doublelift and high tier streamers like boxbox feed their butts off and be pushed out of dozens of CS. Do your best, enjoy the game, it's a game.
See you on the rift!