HMM. First fandom-as-we-generally-know-it was HP, in the middle of 2005! Specifically the Remus/Sirius corner, because the Marauders were way more interesting to me than Harry's generation. I... kind of can't remember how I stumbled into it now? All I remember is discovering fic and then MAINLINING IT because as my personal canon backstory it made so much more sense! xDDD

My first-ever fandom was probably fairytales and folktales, though; I grew up on those and pretty much never stopped reading them. (SF/F reading tendencies, hello!)