Welcome! You're not wrong, but we have multiple threads discussing exactly this. You may have to go back a page or two as most of the discussion around different compositions was closer to classic release. You'll probably want to look through those older threads for lots of good "how to", beyond just figuring out composition.

Really short summary - I'd recommend starting with:
For Alliance, go Pally, Mage, Mage, Mage (or Lock), Priest (aka PaMMMPr or PaMMLPr).
For Horde, go Warrior, Mage, Mage, Mage (or Lock), Priest (aka WMMMP or WMMLP)

https://www.dual-boxing.com/threads/...eveling-guide!
https://www.dual-boxing.com/threads/...ngeon-leveling