The things ill say are extremly hard to put into action but i hope it helps a little :
1.Don't troll just beacuse something isn't going your way (ex. ur teammate picks off meta)
2.Use wards and don't blame jg cuz the enemy jungler ganked you
3.Pick an appropriate champ (ex. if ur team has 4 squishy Champs who can't engage don't go yummi support, go tanky supp such as leona)
4.Ward dragon, herald
5. Build antiheal when enemy has Champs who heal a lot (mundo, soraka, Warwic, swain...)
6.If you're playing jg or mid GANK other lanes don't just perma farm
7.Stop blaming your teammates because u don't have map awareness
there are more tips but they are too advanced so I won't put them here, but these 7 tips are also extremly advanced but i hope it helps someone