Hi, I'm a newer player as well. Level 12, and slowly rising. Here's a list of things I've learned from my live matches so far you may find useful, please excuse the upcoming wall of text.
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The team that controls vision should win. Ward. Ward. Ward. Make it so you can see the jungler gank going, make it so you can keep track of buffs, make
it so you just keep map vision up and don't get caught out in the open. Just...ward. When I'm playing a support, sightstone is my first buy usually, and I keep our bot lane under such a lockdown even the NSA goes goddamn. When I'm top or mid, I buy wards here and there so if I'm getting ganked, I most likely see it coming and can put myself in a better position to safely live through it, or set up a counter gank. I drop wards all the time in lanes where we've lost towers, or enemy jungles, so I can keep vision on the map, not get caught out, or let my team pick off stragglers and then win the ensuing team fight. Wardings not just for supports and junglers, and anyone who says otherwise is bad.
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Watch your other lanes. If you are bot lane and mid is getting crushed, then that means mid will probably roam soon. Very easy to get caught in a 2v3 or even 3v4 if you aren't paying attention. Dominating your lane but your nearby lane is losing? Push the enemy out of your lane, then go help your lane briefly with a gank if you feel like you are in a position to do so. A big problem I had starting out was thinking 'This is my lane, this is all I am responsible for.' No. Keep an eye on your map, do what you can to help the team out if you have a legit opportunity to do so.
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Communicate. Tell people/jungler whats going on in the lane. Tell your team when enemies are missing, or you see/think they are roaming to another lane. Don't let another lane get ganked because they weren't paying attention and just assumed your enemy laner was still in the lane with you. Tell your team where you notice the enemy has placed wards so they can come into lane invisibly, and help out, instead of wasting time. Just communicate game plans.
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Don't be so aggressive. I've been noticing A LOT of people I play are all or nothing when it comes to kills. I'm not amazing, but I do well in the KDA department because I just look to win trades, and take a kill when it presents itself. This was especially prevalent when I was in single digits as people were completely bloodthirsty. I'd just chip, chip, chip, and kill them because they let themselves get into such a horrible position.
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Capitalize on victories. Your lane going great? Tell jungler to tip a different lane and not worry about you. Push/ pushing them out of bot lane? Tell your team and set up a dragon kill the next time you force them out. Take down an inhib? Group up and apply pressure or take other objectives, don't let them just farm until your advantage goes away.
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When lanes are done, for the love of God, group up. It's fine having a person split off to kill a huge creep wave, or get jungle buffs, or stop a split pusher, or even go split push if they are good at it, but so many of my losses right now are coming from 1-2 people just running around the map with no clue (and no vision), getting caught out, killed, and then we lose a 3v5 team fight.
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Try to figure out why you are dying/losing lane and work on not doing that. My first day in League, I averaged something horrible like 1 kill for 2 deaths. I realized I wasn't warding, was overextending, was putting myself in bad situations in team fights, and have been working on not doing that. I'm not going to be a game dominator by any means now, but I kill more than I die, and I contribute more to the games now. The forums were actually really helpful for me making my first few strides in this.
For building, I let TAB be my guide. I have a few core items I get every game, but not really in a set-order, but past that I look at what the enemy team is, and what they are building, for what I need to build. If they are stacking armor, then it means a last whisper is coming. If they have WW and Yi on the team, then I want something like a Thornmail/Randuins/Frozen Heart (etc..) to counter auto-attacks. It sucks, but you will have to go to places like 2ez.gg and just read up on champions, and just get a better idea of how to build because the game won't teach you. Other than a few core items, you shouldn't have a completely set build path.
For laning, go to Youtube. Look up how to freeze a lane, and when to push a lane. I've caught a few people off guard because I end up in a 1v2 lane because we have a jungler and they don't, and I just freeze the lane and starve them for gold/xp, and then take the lane over later. This has helped me greatly as I tend to at worst break even in my laning phase, no matter my match-up.
Tier 1 runes are cheap and good to have. Tier 2 runes are overpriced, don't buy them. Tier 3 runes are sexy, and I want them now. Feel free to add some T1 runes for the slight boost they give you and wait out until you can get T3 runes. Even if you are 'just' starting out with a free long sword or cloth armor at the start of the match, that's better than not having them.
I play with my camera unlocked, but some people don't. I think it's personal choice, but unlocked just feels better to me. Try both, use whichever feels like you play the best with.