[Laine]

Are you planning to write the manga entirely in Japanese? Not to sound painfully discouraging, but I advise against it. Modern-day manga is written using so much slang (which is constantly changing), and grammatical exceptions that I'd say it'd be extremely difficult for a foreigner who doesn't know the language (or hasn't lived in Japan and been accustomed to the casual talk) to write a successful Japanese manga.

For a view of modern-day Japanese culture, I recommend you watch recent j-dramas, perhaps even some slice-of-life anime.