[Laine]

Hire a private tutor or start Japanese classes ASAP.

I have a friend who learned enough Japanese in 1.5 years to win 3rd place in a national Japanese speech contest. After that, he went on to learn Mandarin in about 1 year, and is currently studying Cantonese (he's also going to university in Hong Kong). He went to Okinawa for a month and had no problems communicating.

It's possible, but you need the determination and resources.

So take classes, because you can't rely on teaching yourself; there are plenty of exceptions to grammatical rules, and you'll also need assistance in ensuring the accent (I still can't pronounce 'kenryoku' that well -___-). My friend took Japanese class at Japan Society (maybe there's one in your city), alongside businessmen and other adults; it's a very recommended program.

If you're really serious about it, I also recommend you buy a denshi jishou in the meantime.