PLAY TO NOT DIE!
This is the only thing I could tell someone with confidence when it comes to improving at League:
PLAY TO NOT DIE!
I notice a lot of low ranked players die a lot in their games. It usually comes from chasing, diving, over-extending... basically they want kills. But a lot of times they end up getting killed for a kill, or at best they end up just trading kills. I kill you, you kill me. Getting you're enemy fed is never worth. If you try to not die and focus on not dying, you'll probably do better. Maybe your cs might improve too? Maybe you'll focus on learning trades better, so you always come on top? Maybe you'll learn to ward more often? I mean why die and get your enemy fed?
Going 8-15-0 is not worth in my opinion. I'd rather go 2-0-5, if you know what I mean. Heck 15-15-0 isn't worth either. I mean how many times have you had an ally with a load of kills but deaths too and you lose? Many times.
Edit: After reading many comments down below, I think I haven't explained myself well. I think what I said can be taken many different ways, and some see what I said differently than how I meant it. However, down below there are some comments that are great examples of what I'm talking about. You might notice I've replied to them as well, as I agree with what they're saying or they relate to what I'm talking about.