here's what you had to do before fighting Onyxia, the first raid boss in vanilla. nowadays to do a raid you can just press a button and wait a few minutes and
Ubrs key quest
Classic WoW Attunements and Keys to Unlock Dungeons and Raids - Guides - Wowhead
Jump Runs/Skips Archives - VanillaGuides
Classic WoW - Upper Blackrock Spire Guide (UBRS)
WoW Archivist: Upper Blackrock Spire | Engadget
Attunement to Molten Core World of Warcraft MoP 5.2 (How to get to Molten Core) by
WoW Classic Dungeons: how to run them and what attunements are required | PCGamesN