I watched this last night for a refresher as some of my mates explained it to me ages back when I tried playing with them.
It might be helpful. But I'm like you, I have no idea how the mechanics work in terms of terminoligy like AD carrier, tank (I know what a tank is, but no idea who or how it works in this game) etc etc.
So for me, I've been advised to stick to Ashe, Sivir or Jinx (I will probably buy Jinx as I really enjoyed playing her last night). Which are AD characters... I think, someone will probably correct me and please do. I don't know what I'm doing lol.
Those characters are ranged and go bottom, basically last hit minions for gold and attack only when it's safe. Stick to coop vs AI until level 15 (again, what I've been told).
Other than that, I'm as clueless as you mate haha!