Don’t feel like you have to go to ‘elite’ content and work your tail til it falls off so you can call yourself a “real gamer/WoW player” [What does this even mean?]. Because honestly the reality of it is working your way up there to be with all the “ultra mega buff Chads at the gym” and that’s really it.
But if you’re fine with this and this is something you want and fully have ambitions to reach towards, than this changes the story and I am all for rooting you on.
So I’ll start with the most basic darn thing I wished I had known years ago when I first started getting into Dungeons and Raiding:
Every Dungeon and Raid has a tier.
You see this often as Normal->Heoric->Mythic->Mythic+ [Exception for raids is that LFR (Looking For Raid) counts as a tier, positioned behind Normal].
As you move up the tiers, the ilvl of dropped gear goes up. Going into this content you want to start with running the lowest tier [LFR or Dungeon Finder] and reasonably maxing out your total ilvl with the ilvl that drops in that tier before moving onto the next tier [like Normal if you were doing LFR for example] and doing the same again and gradually moving your way up the ladder.
These drops are your bread and butter. Consider potions and enchantments when working with difficult content, this is especially with doing higher Dungeons and Raid tiers. As far as I’m aware there will be no Legendaries going into Dragonflight, so they’re not mentioned here.
And that’s the most basic “quick beginner’s guide” I can give you.