Since it's the first post, I'll show you some of the earlier j-pop singers I know. Unfortunately, some like The Peanuts are too old for me to show you their music, but another duo has covered their songs. W (Double You) who are a duo of Tsuji Nozomi and Kago Ai, have many of their versions of The Peanuts music. Here are some of those songs:
This song is called Koi no Vacance and it was originally by The Peanuts.
This is probably one of the most famous j-pop songs as I've seen it in tons of different anime and sung by many different people.
Here is another j-rock sounding version of Koi no Fuga. I personally like it, it sounds really interesting and really cool.
This is a PV done by Pink Lady, the original singers of this song, in 2004. The song is called Pepper Keibu.
Hope you guys enjoyed the more earlier j-pop. And I know, it sounds a lot like disco, because back then it WAS disco, kinda.