As someone who was in almost the exact same boat as you, nothing at all can beat an actual class with a good teacher, preferably at a college. The reason being is that a lot of things will be difficult to get your head around in America, but if a teacher drills into your head a word like 10,000, you may still have trouble, but once you reach Japan you'll see since ¥10,000 is basically $100. On the flip side, if you ignore certain things, then when you do anything you will only hear noise and not be able to pick out the words you do know. More importantly, it will help you with pronunciation which is really difficult for some english speakers because there is no accenting of syllables. Also know that this is really ambitious, but I think you can make it to a level where you can converse in a year (how well you can converse will come later).
Now lets throw some insider knowledge to help out. First, early on, Katakana will be very helpful, not only due to the overuse of Wasai-eigo (Japanese English) but also since the trains (at least JR Higashi Nihon) will show names in Katakana as well as Kanji (english too on most). Always remember, Japanese is SOV (or OSV) meaning the verb will ALWAYS be at the end. Don't copy the verbs or tenses that store workers use because it is always far too polite for everyday use. Practice whatever Kanji you are told is common, it will pay off, and don't go choosing what you think is common because that could end badly. Lastly, attitude is EVERYTHING with this sort of deal, many foreigners know more Japanese than me but rarely speak it, whereas I charge in headfirst, friends and older Japanese people will correct you, which is good, you need that, but store workers and coworkers probably wont.
I don't know if you're coming for work or travel (if the latter, get the JR Rail pass, you wont regret that), but staying at Guesthouses, Ryokan, or Hostels can greatly help your speaking ability when you travel, especially if there are few or no other foreigners there. Also from my experience, Kanto area sucks in terms of meeting people. In cities like Osaka, Kyoto, Kobe, the people are usually far more willing to talk to you if you speak Japanese. Tokyo you can probably make friends too, but I live outside Tokyo in an "Industrial City" area, and most of the people on the street wont even say Ohayo, compared to west Japan, that's crazy. However, you may be met with entirely different cases as well.
If you're gonna learn on your own, I can offer up the textbook I learned from in College which is called Contemporary Japanese by Eriko Sato. I learned from Mrs. Sato herself, but I think the book is well enough laid out that all she teaches in her classes (like explanations and whanot) is actually in great detail in the book itself. I'm pretty sure they're up on Amazon. If you have a good base of understanding, then most important will be to just come and speak Japanese, your abilities will grow from there. Sorry I wrote a fucking essay ~_^
