We've all started somewhere, the thing is, you're already better than most people by simply admitting and realising that you're not very good at this game. That doesn't mean to say that you can't be.
My champion that I would encourage you to learn is Azir, he is a very very mechanically demanding champion, learn his abilities and play bot games, learn simple combos like W-Q to harass, learn when to trade in lane (look at the amount of minions, how close you are to their turret, look at the minimap and ward so you know it's safe to do so)
Practice his drift combo (w-e-q) till you can perform it perfectly, watch videos of pro games and watch how they play him.
Once you have a good understanding and mechanical skill, you will then have a champion that can more or less solo carry a late game team fight.
After that, increase your map awareness and use that to your advantage.
for example : you notice you have a slow minion wave building up and heading to their bot side t2 turret, that means someone is going to have to clear it soon, tell your team to start getting vision control over the top side of the map, it'll put pressure on them, you then have the choice to take top/mid/baron, all situational choices.
One other thing to note:
Concentrate on farming minions and avoiding damage rather than kills, with azir at the moment im starting with an RoA build for sustain so I can farm myself up to a nashor then deathcap easily and provides some counter to bursts.
Sorry for the wall of text though, hope something of that was at least useful to someone. I might post some more tips through out the day depending on how bored I get at work.