ok... i would say your not ready for ranked... mainly because you dont even fucking play ranked your last ranked game was rengar 3 days ago and the one after that was fucking 15! OMFG FIFTH-TEEN days ago.
but yeah onto advice.
1 stop playing rengar if you had the skills to polight a champion like rengar... you wouldnt need to play rengar...
2.get 3 champs for your 2 rolls... assassin/damage. so for a jungler this could be master yi whos easy and reliable. next be a bruiser of some kind (you know battle mage what the fuck ever half damage half tank) the jungle is filled with these like jarvan vi xin udyr shyvana ect. but go for an easy one... easy and repeatable is what you want. and next be a tank. you know... sej... amumu... do the same for your 2nd roll.
now dont rush these... dont be like herp de derp tier list find some shit you like... but is also easy and repeatable... i like yi shyvana and sej.i got an 80% winrate on yi and sej and 45% on shyvana and im in plat 2 which is the top 3.2% of players.
so yeah find some stuff you REALLY like.
next step be understanding builds.
when people dont know a build they go "herp de derp he got sunfire so i got sunfire" thats not really how items in league work.
every champion has a set of rules to follow.
for example on master yi its. 2 damage items. 1 armor, 1 jugger naut item and 2 magic resist.
his 2 damage items should always be
and
but its where you get into things like armor it starts getting different because you have choices.

same for juggernaut
(
is also one but dont get on yi due to yis e not scaling with base ad anymore)
so yeah figure out the rules for builds for the champs you want to play.
and the last and final thing is... just spam it.... spam it spam it spam...
the is no "how to get diamond in 5 simple steps" and if the is its STUPIDLY basic shit.
"cs better"
"dont feed"
"ward ur lane"
"dont afk in jungle"
"have an impact"
oh and one last thing... play ranked... iv played more ranked the last 2 days then u have the whole season and pros play what i played the last 3 days in a day