If I want to learn Japanese well enough to live in Japan, how should I start?

Everyone starts Japanese by studying the katakana and hiragana, working through kanji, basic vocabulary and grammar, and works their way to more complex words and forms. The precise methods vary, but the general ideas remain the same.

Your real question seems to be more along the lines of "how much Japanese do I need to know to live in Japan?" That basically depends on how adventurous you are. Some people move there first and learn while in the culture (getting help at first for things like finding an apartment... etc). Others don't feel comfortable until they've earned a graduate degree and married a native speaker.

To put something concrete on this... more or less non-answer, if you're young enough to do an exchange program (high school or college), I HIGHLY recommend you try that route, as you will have people available to help you with cultural difficulties in a case where your language skills aren't up to the task.

thanks!