Help with teamfighting?

Teamfighting is a dance of switching between priority target to priority target. When you're in solo queue, you usually have to decide on your own who the priority target is from one given moment to the other, but when you play on a team and have a designated teamfight shotcaller, they'll tell the team who to focus in the fight

That's a big task. Depending on your champ. Some can engage early and some engage later. Also need to focus there strong suit and disable it so your team can have more control. It's basically about objectives which comes with communication and experience.

try out all champs for yourself so you know what to expect when facing them.

read lol wiki pages thoroughly

to start, study the top 20-30 most popular champs in your division.