New player, just reached level 40. How the fuck do you start to learn this game?

Yo, so me and my brother started playing several days ago, and we both reached 40 recently. I was, originally, a human holy paladin as a healer, but after being told in multiple dungeons that that is a useless class, and feeling way too much pressure being both new and a healer (so many kicks), switched to ret pally and dps.But now I'm reading in places that ret pally is kind of worthless. Regardless, I soldiered on (because I've sunk about 20 hours into this char, despite being told that apparently 40 can be reached in like an hour), and tried watching some youtube videos on how to git gud as a retpal, but... god damn, nothing they said made any sense to me.I'm worried because as we get to higher levels it seems that no one in dungeons (and I'd imagine raids, though does anyone do the low level raids anymore?) is gonna let new player's shit slide anymore. Already we've been kicked from level 20 dungeons because we don't know what we're doing and people seem to have a low tolerance for that, given the game is so old, but fuckin' hell, everytime I learn something it seems I've barely scratched the surface.TL;DR: Me and a friend are new and just doing quests and dungeons, but get kicked from dungeons on occasion for being noobs. Seeking advice on how to git gud and whether or not ret pally is worthless.EDIT: Thanks for all the very helpful and encouraging responses! I wish the people I'd met online so far were as friendly and helpful as you lot.