1) Pickup a book like "Japanese in 10 Minutes a Day"
http://www.amazon.com/exec/obidos/tg/detail/-/0944502369/qid=1128458967/sr=1-1/102-8222791-8891352?v=glance&s=booksISBN: 0944502369
You will want to get your head wrapped around how to create sentences and the backwards verb placement as soon as possible.
The grammatical differences will get you caught up quite easily until you've master some of it, and it's better to get this estabilished well before using Kanji/hiragana/etc.
Also, keep in mind that once you've learned how to say something in a sentence properly, the various writing methods merely are yet-another-way to write the same thing in many ways. (eg. you can write the same Japanese sentence in romanji/Kanji/hiragana and even katakana, and any native will be able to read it and understand you)
2) Once you're at the stage where you can blurt out the basics like "Hi! Could you tell me where the ATM is?" or "I'd like a bowl of Ramen please", you're ready for the next part - pronounciation.
"Pronounce It Perfectly in Japanese "
http://www.amazon.com/exec/obidos/tg/detail/-/0812080351/qid=1128459212/sr=1-2/102-8222791-8891352?v=glance&s=booksISBN 0812080351
This guide will go over each and every one of the sounds in Japanese in depth, and allow you to practice each for perfectly (as well as you can as isolate as you are now). This is a =key= to being understood - trust me, if you blurt it out English style, you won't be understood! The key to being understood well in Japanese, even with basic sentences, is the proper use of Japanese innotation and pronounciation. An excellent reference for any learner in fact.
3) Pick a good college level textbook that has audio cassettes/CDs.
My pick is "Japanese for Everyone"
http://www.amazon.com/exec/obidos/tg/detail/-/0870408534/qid=1128459399/sr=2-1/102-8222791-8891352?v=glance&s=booksISBN: 0870408534
Other two picks many go with are:
Japanese for Busy People series
(ISBN 4770018827 etc)
and
Yookoso!
ISBN 0072354283
I prefer both Japanese for Everyone and Yookoso! because they're a one-volume college level text that will cover a standard Japanese 101-103 (full year of study) in one book. Japanese for Busy People goes a lot slower, and is broken up into numerous volumes (silly, IMO - same thing, less dense coverage, and more money for you to spend) and is not a text I prefer despite others saying good.
Both JFE and Y have audio cassettes/CDs you can buy to really listen and practice understanding with the numerous practice lessons they have. They also have all of the answers for a self-learner in the back.
If you're good at it, you can cover the entire book in 6 months to a year, and be at a decent level for further lessons in person. Keep in mind that they will not get you the necessary in-person practice needed to really make your Japanese good, only far enough to know the basics.
JFE also has a Kanji Workbook and Workbook that will make sure you cover the basics and learn them well.
4) If you can get a copy of the few Japanese video language lessons out there - these will help you see it in action, and learn faster.
5) Don't forget to pickup the hiragana/katakana fast once you're past #1 above:
Let's Learn Hiragana
0870117092
This is so basic, you'll need it for any more Japanese you learn.
6) First Up Japanese is a book that will cover in very compact book, that's super easy to read (once you know a bit of Japanese) all that you should know and have covered in the first year or so. You may need to get this imported from japan, but a good reference no matter what. Many will be surprised that such basic topics covered will be hard to pass in the basic tests they have unless you know it well.
7) Daily use of
http://www.popjisyo.com/WebHint/Portal_e.aspxand
http://honyaku.yahoo.co.jp/urlon Japanese news websites and other sites of interest.
The more you see the Japanese (and translation) together, the faster you'll figure out what goes where.
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Aside from that, the fastest way is to have a Japanese speaker tutor you.