Hey novice, nice seeing some interest.
Just a small preamble sprouting from what I've noticed a lot these days: I've made it a rule for myself to take what people say on j-music forums with a grain of salt when it comes to suggestions, because I think a lot of j-music fans these days gravitate to specific genres of the music as a whole, so you'll see more emphasis on those even though it doesn't always reflect the diversity of the music scene (I have my own biases, of course). I've been trying to explore out of the standard pop and rock genres, but it can be hard with the lack of info.
Okay, let's see...
For some pop metal, try X Japan. I didn't like them at first, but they grow on you, I guess.
For some good mainstream rock with wonderful ranges of styles, try L'arc en Ciel or Gackt. Info on them is pretty easy to find.
Other good rock bands are: Luna Sea or Siam Shade
For some R&B, try Utada Hikaru.
For some dark, hard rock, try Dir en Grey.
For some sexy, sexy jazz, try Paris Match:
http://nippop.com/artist/artist_id-78/artist_name-paris_match/
For some Japanese rock music with a Western flavor, but still skillfully written, try FAKE? or Ken Lloyd's former band, Oblivion Dust.
For punk rock, try Kuroyume or SADS (same vocalist)
For some gothic synth pseudo-classical music, try Malice Mizer.
For some adult alternative music, try Spitz.
For some beautifully written music with a huge range of styles and beautiful composition, try Yoko Kanno, either with or without Maaya Sakamoto. Okay, that's not a genre, but Kanno kinda defies genre.
http://nippop.com/artist/artist_id-133/artist_name-yoko_kanno/
I've been getting into Yae, whose three albums I heard (Blue Line, Flowing to the Sky, and Aloha Nui), seems to indicate an interest in different cultural-traditional music styles.
Hm... that's all I can think of. Lemme know if you find something of interest, and good luck!