Actually, I suppose Hamasaki was my first Japanese artist as well. I used to follow some webcomics on Drunkduck, and someone embedded an audio player on theirs. That was how I found "Trauma" and started from there XD
For me, the first song I heard was Angel's Song, which was embedded at the time on a blog or...something. Either way, I always loved it, and kept replaying it and trying to learn the words and stuff. I obsessed, found more music and just kept going from there.