Your first step is to enroll in a university. I would recommend one that has Japanese classes (doesn't have to have a Japanese major, just classes at least). Most larger universities have study abroad programs to Japan, and there's no better way to learn Japanese and experience daily life in Japan than to study abroad. Just like you said; complete immersion is the key. I studied abroad in Japan for one year (my junior year of college), and it was well worth it... one of the best times in my life. Right now I'm going on to my senior year of college back here in the US and am applying for the JET Program, so that I may go back to Japan (for good this time!).
As for working in Japan, teaching English is sort of the ticket into Japan, and once you're there and settled you can look into moving on to a real career after you're done teaching (or you can continue teaching if that's something you enjoy
) Like MMM said, you can't just go to Japan and try to apply for a job. You have to apply before going to Japan so you can get a proper working visa. The JET Programme is good for this, because the hiring and interviewing is done right in the Japanese consulates here in the US (and in other countries). The JET Programme also doesn't really require much aside from a degree and a willingness to live in Japanese society. Japanese language skills do help, but they aren't necessary for the job. In my own opinion though, it is still best to become at least semi-fluent in the language before trying to live here - this is where the studying abroad came in for me, as I learned so much more Japanese by studying in Japan than I did before.
Good luck!