An area I would love to improve on, team composition, how do I learn about this?

Its like this, do champs work together well? Would a vlad and lb go together well? No. Because lb is an assassin and bursts people. Vlad is aoe/sustain. Would a malph/ori work together well? Yes. Because ori can ball malph then malph ults in and ori ults. Champs that work together well is what makes a comp. If none of the champs on your team work together well you probably arent going to win unless the other team is the same way. Team comps win team fights

The LCS casters do a great job of explaining team comps in champion select. I'd also take a look at other regions such as Korea or China because they tend to many more different kinds of team compositions.

On a cursory glance, you can look at the objectives of certain champs and see if you should either balance or contribute. If you have J4 Malph Ori MF, you can balance with Janna (peel) or contribute with Leona (Engage/peel).