Back in about 1999 I was in my Pokemon/Rocketshipping phase and I got my dad to buy a Japanese Team Rocket CD drama off the internet.
I listened to it, it had some cool songs (that I still enjoy today) on it and the drama was pretty fun from what I could understand at the time.
As I got more into anime (1999-2000) I started downloading japanese DBZ and Sailor Moon songs and listened to them. That was my first "experience" with japanese music and I downloaded songs by artists that had performed DBZ OP and ED songs (like ZARD, Hironobu kageyama and Field of View).
Then I started visiting J-music blogs and downloading more zany tracks like Gackt's Vanilla and deeper tracks like Ophelia by L'arc. Some Chinese friends gave me burned C-pop CDs but I've lost them
Then one day in 2003/4 a friend of a friend of a kind-of-a friend lent me his Ayumi CDs - RAINBOW and Ayu Trance 2. He didn't speak a word of japanese but loved her voice and trance music. I borrowed them, took them home and listened.
Being a fan of trance music I listened to AT2 first and then RAINBOW. Then I burned copies of them and started looking for more info on her. At this point I couldn't afford CDs and importing so I downloaded tracks here and there and got more music from sites with anime soundtracks and tried other artists. I really liked Utada's simple and clean when I downloaded it in about 05? (can't remember when it came out lol).
In about 2005 I discovered AHS and joined ^_^ From here I've had access to tonnes of bands and artists I've never heard of as well as those whose tracks I downloaded in the "early days".
Anime OPs and EDs were what bought me to J-pop but I'm certain that it's Ayu who really captured my interests.