Practice practice practice, there are youtube videos where you can try and hone your csing. Cs is very important and you need to know when to engage in teamfights and when laning, when you can kill them with a combo. If youre not confident in your all in-ing then play champs who are late game and farm. Try lolking for guides, they go in detail and helped me a lot.
Poke and cs with bouncing blade thing with sivir. Take that first always on her to get a cs lead.
With rengar, you need to get rangers trailblazer for clear speed and then get tiamat. Once you hit 6, you need to look at your map all the time for when lanes are pushed up. If not, then farm super hard to finish ravenous and trade trailblazer with chilling smite and get warrior.
Best of luck on ekko, i dont know much about him and i just dont think he is all that good right now. So youtube that (: